Synchronous Serial TransmissionSynchronous serial transmission requires that the sender
and receiver share a clock with one another, or that the
sender provide a strobe or other timing signal so that the
receiver knows when to read the next bit of
the data. In most forms of serial Synchronous
communication, if there is no data available at a given
instant to transmit, a fill character must be sent instead
so that data is always being transmitted. Synchronous
communication is usually more efficient because only data
bits are transmitted between sender and receiver, and
synchronous communication can be more costly if extra wiring
and circuits are required to share a clock signal between
the sender and receiver.A form of Synchronous transmission is used with printers
and fixed disk devices in that the data is sent on one set
of wires while a clock or strobe is sent on a different
wire. Printers and fixed disk devices are not normally
serial devices because most fixed disk interface standards
send an entire word of data for each clock or strobe signal
by using a separate wire for each bit of the word. In the
PC industry, these are known as Parallel devices.The standard serial communications hardware in the PC
does not support Synchronous operations. This mode is
described here for comparison purposes only.Asynchronous Serial TransmissionAsynchronous transmission allows data to be transmitted
without the sender having to send a clock signal to the
receiver. Instead, the sender and receiver must agree on
timing parameters in advance and special bits are added to
each word which are used to synchronize the sending and
receiving units.When a word is given to the UART for Asynchronous
transmissions, a bit called the "Start Bit" is added to the
beginning of each word that is to be transmitted. The Start
Bit is used to alert the receiver that a word of data is
about to be sent, and to force the clock in the receiver
into synchronization with the clock in the transmitter.
These two clocks must be accurate enough to not have the
frequency drift by more than 10% during the transmission of
the remaining bits in the word. (This requirement was set
in the days of mechanical teleprinters and is easily met by
modern electronic equipment.)After the Start Bit, the individual bits of the word of
data are sent, with the Least Significant Bit (LSB) being
sent first. Each bit in the transmission is transmitted for
exactly the same amount of time as all of the other bits,
and the receiver looks at the wire at
approximately halfway through the period assigned to each
bit to determine if the bit is a 1 or a
0. For example, if it takes two seconds
to send each bit, the receiver will examine the signal to
determine if it is a 1 or a
0 after one second has passed, then it
will wait two seconds and then examine the value of the next
bit, and so on.The sender does not know when the receiver has
looked at the value of the bit. The sender
only knows when the clock says to begin transmitting the
next bit of the word.When the entire data word has been sent, the transmitter
may add a Parity Bit that the transmitter generates. The
Parity Bit may be used by the receiver to perform simple
error checking. Then at least one Stop Bit is sent by the
transmitter.When the receiver has received all of the bits in the
data word, it may check for the Parity Bits (both sender and
receiver must agree on whether a Parity Bit is to be used),
and then the receiver looks for a Stop Bit. If the Stop Bit
does not appear when it is supposed to, the UART considers
the entire word to be garbled and will report a Framing
Error to the host processor when the data word is read. The
usual cause of a Framing Error is that the sender and
receiver clocks were not running at the same speed, or that
the signal was interrupted.Regardless of whether the data was received correctly or
not, the UART automatically discards the Start, Parity and
Stop bits. If the sender and receiver are configured
identically, these bits are not passed to the host.If another word is ready for transmission, the Start Bit
for the new word can be sent as soon as the Stop Bit for the
previous word has been sent.Because asynchronous data is self
synchronizing, if there is no data to transmit, the
transmission line can be idle.Other UART FunctionsIn addition to the basic job of converting data from
parallel to serial for transmission and from serial to
parallel on reception, a UART will usually provide
additional circuits for signals that can be used to indicate
the state of the transmission media, and to regulate the
flow of data in the event that the remote device is not
prepared to accept more data. For example, when the device
connected to the UART is a modem, the modem may report the
presence of a carrier on the phone line while the computer
may be able to instruct the modem to reset itself or to not
take calls by raising or lowering one more of these
extra signals. The function of each of these additional
signals is defined in the EIA RS232-C standard.The RS232-C and V.24 StandardsIn most computer systems, the UART is connected to
circuitry that generates signals that comply with the EIA
RS232-C specification. There is also a CCITT standard named
V.24 that mirrors the specifications included in
RS232-C.RS232-C Bit Assignments (Marks and Spaces)In RS232-C, a value of 1 is called
a Mark and a value of
0 is called a Space.
When a communication line is idle, the line is said to be
Marking, or transmitting continuous
1 values.The Start bit always has a value of
0 (a Space). The Stop Bit always has a
value of 1 (a Mark). This means that
there will always be a Mark (1) to Space (0) transition on
the line at the start of every word, even when multiple
word are transmitted back to back. This guarantees that
sender and receiver can resynchronize their clocks
regardless of the content of the data bits that are being
transmitted.The idle time between Stop and Start bits does not
have to be an exact multiple (including zero) of the bit
rate of the communication link, but most UARTs are
designed this way for simplicity.In RS232-C, the "Marking" signal (a
1) is represented by a voltage between
-2 VDC and -12 VDC, and a "Spacing" signal (a
0) is represented by a voltage between
0 and +12 VDC. The transmitter is supposed to send +12
VDC or -12 VDC, and the receiver is supposed to allow for
some voltage loss in long cables. Some transmitters in
low power devices (like portable computers) sometimes use
only +5 VDC and -5 VDC, but these values are still
acceptable to a RS232-C receiver, provided that the cable
lengths are short.RS232-C Break SignalRS232-C also specifies a signal called a
Break, which is caused by sending
continuous Spacing values (no Start or Stop bits). When
there is no electricity present on the data circuit, the
line is considered to be sending
Break.The Break signal must be of a
duration longer than the time it takes to send a complete
byte plus Start, Stop and Parity bits. Most UARTs can
distinguish between a Framing Error and a Break, but if
the UART cannot do this, the Framing Error detection can
be used to identify Breaks.In the days of teleprinters, when numerous printers
around the country were wired in series (such as news
services), any unit could cause a Break
by temporarily opening the entire circuit so that no
current flowed. This was used to allow a location with
urgent news to interrupt some other location that was
currently sending information.In modern systems there are two types of Break
signals. If the Break is longer than 1.6 seconds, it is
considered a "Modem Break", and some modems can be
programmed to terminate the conversation and go on-hook or
enter the modems' command mode when the modem detects this
signal. If the Break is smaller than 1.6 seconds, it
signifies a Data Break and it is up to the remote computer
to respond to this signal. Sometimes this form of Break
is used as an Attention or Interrupt signal and sometimes
is accepted as a substitute for the ASCII CONTROL-C
character.Marks and Spaces are also equivalent to
Holes and No Holes in paper
tape systems.Breaks cannot be generated from paper tape or from
any other byte value, since bytes are always sent with
Start and Stop bit. The UART is usually capable of
generating the continuous Spacing signal in response to
a special command from the host processor.RS232-C DTE and DCE DevicesThe RS232-C specification defines two types of
equipment: the Data Terminal Equipment (DTE) and the Data
Carrier Equipment (DCE). Usually, the DTE device is the
terminal (or computer), and the DCE is a modem. Across
the phone line at the other end of a conversation, the
receiving modem is also a DCE device and the computer that
is connected to that modem is a DTE device. The DCE
device receives signals on the pins that the DTE device
transmits on, and vice versa.When two devices that are both DTE or both DCE must be
connected together without a modem or a similar media
translator between them, a NULL modem must be used. The
NULL modem electrically re-arranges the cabling so that
the transmitter output is connected to the receiver input
on the other device, and vice versa. Similar translations
are performed on all of the control signals so that each
device will see what it thinks are DCE (or DTE) signals
from the other device.The number of signals generated by the DTE and DCE
devices are not symmetrical. The DTE device generates
fewer signals for the DCE device than the DTE device
receives from the DCE.RS232-C Pin AssignmentsThe EIA RS232-C specification (and the ITU equivalent,
V.24) calls for a twenty-five pin connector (usually a
DB25) and defines the purpose of most of the pins in that
connector.In the IBM Personal Computer and similar systems, a
subset of RS232-C signals are provided via nine pin
connectors (DB9). The signals that are not included on
the PC connector deal mainly with synchronous operation,
and this transmission mode is not supported by the UART
that IBM selected for use in the IBM PC.Depending on the computer manufacturer, a DB25, a DB9,
or both types of connector may be used for RS232-C
communications. (The IBM PC also uses a DB25 connector
for the parallel printer interface which causes some
confusion.)Below is a table of the RS232-C signal assignments in
the DB25 and DB9 connectors.DB25 RS232-C PinDB9 IBM PC
PinEIA Circuit SymbolCCITT Circuit SymbolCommon
NameSignal SourceDescription1-AA101PG/FG-Frame/Protective Ground23BA103TDDTETransmit Data32BB104RDDCEReceive Data47CA105RTSDTERequest to Send58CB106CTSDCEClear to Send66CC107DSRDCEData Set Ready75AV102SG/GND-Signal Ground81CF109DCD/CDDCEData Carrier Detect9-----Reserved for Test10-----Reserved for Test11-----Reserved for Test12-CI122SRLSDDCESec. Recv. Line Signal Detector13-SCB121SCTSDCESecondary Clear to Send14-SBA118STDDTESecondary Transmit Data15-DB114TSETDCETrans. Sig. Element Timing16-SBB119SRDDCESecondary Received Data17-DD115RSETDCEReceiver Signal Element Timing18--141LOOPDTELocal Loopback19-SCA120SRSDTESecondary Request to Send204CD108.2DTRDTEData Terminal Ready21---RDLDTERemote Digital Loopback229CE125RIDCERing Indicator23-CH111DSRSDTEData Signal Rate Selector24-DA113TSETDTETrans. Sig. Element Timing25--142-DCETest ModeBits, Baud and SymbolsBaud is a measurement of transmission speed in
asynchronous communication. Because of advances in modem
communication technology, this term is frequently misused
when describing the data rates in newer devices.Traditionally, a Baud Rate represents the number of bits
that are actually being sent over the media, not the amount
of data that is actually moved from one DTE device to the
other. The Baud count includes the overhead bits Start, Stop
and Parity that are generated by the sending UART and
removed by the receiving UART. This means that seven-bit
words of data actually take 10 bits to be completely
transmitted. Therefore, a modem capable of moving 300 bits
per second from one place to another can normally only move
30 7-bit words if Parity is used and one Start and Stop bit
are present.If 8-bit data words are used and Parity bits are also
used, the data rate falls to 27.27 words per second, because
it now takes 11 bits to send the eight-bit words, and the
modem still only sends 300 bits per second.The formula for converting bytes per second into a baud
rate and vice versa was simple until error-correcting modems
came along. These modems receive the serial stream of bits
from the UART in the host computer (even when internal
modems are used the data is still frequently serialized) and
converts the bits back into bytes. These bytes are then
combined into packets and sent over the phone line using a
Synchronous transmission method. This means that the Stop,
Start, and Parity bits added by the UART in the DTE (the
computer) were removed by the modem before transmission by
the sending modem. When these bytes are received by the
remote modem, the remote modem adds Start, Stop and Parity
bits to the words, converts them to a serial format and then
sends them to the receiving UART in the remote computer, who
then strips the Start, Stop and Parity bits.The reason all these extra conversions are done is so
that the two modems can perform error correction, which
means that the receiving modem is able to ask the sending
modem to resend a block of data that was not received with
the correct checksum. This checking is handled by the
modems, and the DTE devices are usually unaware that the
process is occurring.By striping the Start, Stop and Parity bits, the
additional bits of data that the two modems must share
between themselves to perform error-correction are mostly
concealed from the effective transmission rate seen by the
sending and receiving DTE equipment. For example, if a
modem sends ten 7-bit words to another modem without
including the Start, Stop and Parity bits, the sending modem
will be able to add 30 bits of its own information that the
receiving modem can use to do error-correction without
impacting the transmission speed of the real data.The use of the term Baud is further confused by modems
that perform compression. A single 8-bit word passed over
the telephone line might represent a dozen words that were
transmitted to the sending modem. The receiving modem will
expand the data back to its original content and pass that
data to the receiving DTE.Modern modems also include buffers that allow the rate
that bits move across the phone line (DCE to DCE) to be a
different speed than the speed that the bits move between
the DTE and DCE on both ends of the conversation. Normally
the speed between the DTE and DCE is higher than the DCE to
DCE speed because of the use of compression by the
modems.Because the number of bits needed to describe a byte
varied during the trip between the two machines plus the
differing bits-per-seconds speeds that are used present on
the DTE-DCE and DCE-DCE links, the usage of the term Baud to
describe the overall communication speed causes problems and
can misrepresent the true transmission speed. So Bits Per
Second (bps) is the correct term to use to describe the
transmission rate seen at the DCE to DCE interface and Baud
or Bits Per Second are acceptable terms to use when a
connection is made between two systems with a wired
connection, or if a modem is in use that is not performing
error-correction or compression.Modern high speed modems (2400, 9600, 14,400, and
19,200bps) in reality still operate at or below 2400 baud,
or more accurately, 2400 Symbols per second. High speed
modem are able to encode more bits of data into each Symbol
using a technique called Constellation Stuffing, which is
why the effective bits per second rate of the modem is
higher, but the modem continues to operate within the
limited audio bandwidth that the telephone system provides.
Modems operating at 28,800 and higher speeds have variable
Symbol rates, but the technique is the same.The IBM Personal Computer UARTStarting with the original IBM Personal Computer, IBM
selected the National Semiconductor INS8250 UART for use in
the IBM PC Parallel/Serial Adapter. Subsequent generations
of compatible computers from IBM and other vendors continued
to use the INS8250 or improved versions of the National
Semiconductor UART family.National Semiconductor UART Family TreeThere have been several versions and subsequent
generations of the INS8250 UART. Each major version is
described below.INS8250 -> INS8250B
\
\
\-> INS8250A -> INS82C50A
\
\
\-> NS16450 -> NS16C450
\
\
\-> NS16550 -> NS16550A -> PC16550DINS8250This part was used in the original IBM PC and
IBM PC/XT. The original name for this part was the
INS8250 ACE (Asynchronous Communications Element)
and it is made from NMOS technology.The 8250 uses eight I/O ports and has a one-byte
send and a one-byte receive buffer. This original
UART has several race conditions and other
flaws. The original IBM BIOS includes code to work
around these flaws, but this made the BIOS dependent
on the flaws being present, so subsequent parts like
the 8250A, 16450 or 16550 could not be used in the
original IBM PC or IBM PC/XT.INS8250-BThis is the slower speed of the INS8250 made
from NMOS technology. It contains the same problems
as the original INS8250.INS8250AAn improved version of the INS8250 using XMOS
technology with various functional flaws
corrected. The INS8250A was used initially in PC
clone computers by vendors who used
clean BIOS designs. Because of the
corrections in the chip, this part could not be used
with a BIOS compatible with the INS8250 or
INS8250B.INS82C50AThis is a CMOS version (low power consumption)
of the INS8250A and has similar functional
characteristics.NS16450Same as NS8250A with improvements so it can be
used with faster CPU bus designs. IBM used this
part in the IBM AT and updated the IBM BIOS to no
longer rely on the bugs in the INS8250.NS16C450This is a CMOS version (low power consumption)
of the NS16450.NS16550Same as NS16450 with a 16-byte send and receive
buffer but the buffer design was flawed and could
not be reliably be used.NS16550ASame as NS16550 with the buffer flaws
corrected. The 16550A and its successors have become
the most popular UART design in the PC industry,
mainly due to its ability to reliably handle higher
data rates on operating systems with sluggish
interrupt response times.NS16C552This component consists of two NS16C550A CMOS
UARTs in a single package.PC16550DSame as NS16550A with subtle flaws
corrected. This is revision D of the 16550 family
and is the latest design available from National
Semiconductor.The NS16550AF and the PC16550D are the same thingNational reorganized their part numbering system a few
years ago, and the NS16550AFN no longer exists by that
name. (If you have a NS16550AFN, look at the date code on
the part, which is a four digit number that usually starts
with a nine. The first two digits of the number are the
year, and the last two digits are the week in that year
when the part was packaged. If you have a NS16550AFN, it
is probably a few years old.)The new numbers are like PC16550DV, with minor
differences in the suffix letters depending on the package
material and its shape. (A description of the numbering
system can be found below.)It is important to understand that in some stores, you
may pay $15(US) for a NS16550AFN made in 1990 and in
the next bin are the new PC16550DN parts with minor fixes
that National has made since the AFN part was in
production, the PC16550DN was probably made in the past
six months and it costs half (as low as $5(US) in
volume) as much as the NS16550AFN because they are readily
available.As the supply of NS16550AFN chips continues to shrink,
the price will probably continue to increase until more
people discover and accept that the PC16550DN really has
the same function as the old part number.National Semiconductor Part Numbering SystemThe older NSnnnnnrqp part
numbers are now of the format
PCnnnnnrgp.The r is the revision
field. The current revision of the 16550 from National
Semiconductor is D.The p is the package-type
field. The types are:"F"QFP(quad flat pack) L lead type"N"DIP(dual inline package) through hole straight lead
type"V"LPCC(lead plastic chip carrier) J lead typeThe g is the product grade
field. If an I precedes the
package-type letter, it indicates an
industrial grade part, which has higher
specs than a standard part but not as high as Military
Specification (Milspec) component. This is an optional
field.So what we used to call a NS16550AFN (DIP Package) is
now called a PC16550DN or PC16550DIN.Other Vendors and Similar UARTsOver the years, the 8250, 8250A, 16450 and 16550 have
been licensed or copied by other chip vendors. In the case
of the 8250, 8250A and 16450, the exact circuit (the
megacell) was licensed to many vendors,
including Western Digital and Intel. Other vendors
reverse-engineered the part or produced emulations that had
similar behavior.In internal modems, the modem designer will frequently
emulate the 8250A/16450 with the modem microprocessor, and
the emulated UART will frequently have a hidden buffer
consisting of several hundred bytes. Because of the size of
the buffer, these emulations can be as reliable as a 16550A
in their ability to handle high speed data. However, most
operating systems will still report that the UART is only a
8250A or 16450, and may not make effective use of the extra
buffering present in the emulated UART unless special
drivers are used.Some modem makers are driven by market forces to abandon
a design that has hundreds of bytes of buffer and instead
use a 16550A UART so that the product will compare favorably
in market comparisons even though the effective performance
may be lowered by this action.A common misconception is that all parts with
16550A written on them are identical in
performance. There are differences, and in some cases,
outright flaws in most of these 16550A clones.When the NS16550 was developed, the National
Semiconductor obtained several patents on the design and
they also limited licensing, making it harder for other
vendors to provide a chip with similar features. Because of
the patents, reverse-engineered designs and emulations had
to avoid infringing the claims covered by the patents.
Subsequently, these copies almost never perform exactly the
same as the NS16550A or PC16550D, which are the parts most
computer and modem makers want to buy but are sometimes
unwilling to pay the price required to get the genuine
part.Some of the differences in the clone 16550A parts are
unimportant, while others can prevent the device from being
used at all with a given operating system or driver. These
differences may show up when using other drivers, or when
particular combinations of events occur that were not well
tested or considered in the &windows; driver. This is because
most modem vendors and 16550-clone makers use the Microsoft
drivers from &windows; for Workgroups 3.11 and the µsoft;
&ms-dos; utility as the primary tests for compatibility with
the NS16550A. This over-simplistic criteria means that if a
different operating system is used, problems could appear
due to subtle differences between the clones and genuine
components.National Semiconductor has made available a program
named COMTEST that performs
compatibility tests independent of any OS drivers. It
should be remembered that the purpose of this type of
program is to demonstrate the flaws in the products of the
competition, so the program will report major as well as
extremely subtle differences in behavior in the part being
tested.In a series of tests performed by the author of this
document in 1994, components made by National Semiconductor,
TI, StarTech, and CMD as well as megacells and emulations
embedded in internal modems were tested with COMTEST. A
difference count for some of these components is listed
below. Because these tests were performed in 1994, they may
not reflect the current performance of the given product
from a vendor.It should be noted that COMTEST normally aborts when an
excessive number or certain types of problems have been
detected. As part of this testing, COMTEST was modified so
that it would not abort no matter how many differences were
encountered.VendorPart NumberErrors (aka "differences" reported)National(PC16550DV)0National(NS16550AFN)0National(NS16C552V)0TI(TL16550AFN)3CMD(16C550PE)19StarTech(ST16C550J)23RockwellReference modem with internal 16550 or an
emulation (RC144DPi/C3000-25)117SierraModem with an internal 16550
(SC11951/SC11351)91To date, the author of this document has not found any
non-National parts that report zero differences using the
COMTEST program. It should also be noted that National
has had five versions of the 16550 over the years and the
newest parts behave a bit differently than the classic
NS16550AFN that is considered the benchmark for
functionality. COMTEST appears to turn a blind eye to the
differences within the National product line and reports
no errors on the National parts (except for the original
16550) even when there are official erratas that describe
bugs in the A, B and C revisions of the parts, so this
bias in COMTEST must be taken into account.It is important to understand that a simple count of
differences from COMTEST does not reveal a lot about what
differences are important and which are not. For example,
about half of the differences reported in the two modems
listed above that have internal UARTs were caused by the
clone UARTs not supporting five- and six-bit character
modes. The real 16550, 16450, and 8250 UARTs all support
these modes and COMTEST checks the functionality of these
modes so over fifty differences are reported. However,
almost no modern modem supports five- or six-bit characters,
particularly those with error-correction and compression
capabilities. This means that the differences related to
five- and six-bit character modes can be discounted.Many of the differences COMTEST reports have to do with
timing. In many of the clone designs, when the host reads
from one port, the status bits in some other port may not
update in the same amount of time (some faster, some slower)
as a real NS16550AFN and COMTEST looks
for these differences. This means that the number of
differences can be misleading in that one device may only
have one or two differences but they are extremely serious,
and some other device that updates the status registers
faster or slower than the reference part (that would
probably never affect the operation of a properly written
driver) could have dozens of differences reported.COMTEST can be used as a screening tool to alert the
administrator to the presence of potentially incompatible
components that might cause problems or have to be handled
as a special case.If you run COMTEST on a 16550 that is in a modem or a
modem is attached to the serial port, you need to first
issue a ATE0&W command to the modem so that the modem
will not echo any of the test characters. If you forget to
do this, COMTEST will report at least this one
difference:Error (6)...Timeout interrupt failed: IIR = c1 LSR = 618250/16450/16550 RegistersThe 8250/16450/16550 UART occupies eight contiguous I/O
port addresses. In the IBM PC, there are two defined
locations for these eight ports and they are known
collectively as COM1 and COM2. The makers of PC-clones and
add-on cards have created two additional areas known as COM3
and COM4, but these extra COM ports conflict with other
hardware on some systems. The most common conflict is with
video adapters that provide IBM 8514 emulation.COM1 is located from 0x3f8 to 0x3ff and normally uses
IRQ 4. COM2 is located from 0x2f8 to 0x2ff and normally uses
IRQ 3. COM3 is located from 0x3e8 to 0x3ef and has no
standardized IRQ. COM4 is located from 0x2e8 to 0x2ef and has
no standardized IRQ.A description of the I/O ports of the 8250/16450/16550
UART is provided below.I/O PortAccess AllowedDescription+0x00write (DLAB==0)Transmit Holding Register
(THR).Information written to this port are
treated as data words and will be transmitted by the
UART.+0x00read (DLAB==0)Receive Buffer Register (RBR).Any
data words received by the UART form the serial link are
accessed by the host by reading this
port.+0x00write/read (DLAB==1)Divisor Latch LSB (DLL)This value
will be divided from the master input clock (in the IBM
PC, the master clock is 1.8432MHz) and the resulting
clock will determine the baud rate of the UART. This
register holds bits 0 thru 7 of the
divisor.+0x01write/read (DLAB==1)Divisor Latch MSB (DLH)This value
will be divided from the master input clock (in the IBM
PC, the master clock is 1.8432MHz) and the resulting
clock will determine the baud rate of the UART. This
register holds bits 8 thru 15 of the
divisor.+0x01write/read (DLAB==0)Interrupt Enable Register
(IER)The 8250/16450/16550 UART
classifies events into one of four categories.
Each category can be configured to generate an
interrupt when any of the events occurs. The
8250/16450/16550 UART generates a single external
interrupt signal regardless of how many events in
the enabled categories have occurred. It is up to
the host processor to respond to the interrupt and
then poll the enabled interrupt categories
(usually all categories have interrupts enabled)
to determine the true cause(s) of the
interrupt.Bit 7Reserved, always 0.Bit 6Reserved, always 0.Bit 5Reserved, always 0.Bit 4Reserved, always 0.Bit 3Enable Modem Status Interrupt (EDSSI). Setting
this bit to "1" allows the UART to generate an
interrupt when a change occurs on one or more of the
status lines.Bit 2Enable Receiver Line Status Interrupt (ELSI)
Setting this bit to "1" causes the UART to generate
an interrupt when the an error (or a BREAK signal)
has been detected in the incoming data.Bit 1Enable Transmitter Holding Register Empty
Interrupt (ETBEI) Setting this bit to "1" causes the
UART to generate an interrupt when the UART has room
for one or more additional characters that are to be
transmitted.Bit 0Enable Received Data Available Interrupt
(ERBFI) Setting this bit to "1" causes the UART to
generate an interrupt when the UART has received
enough characters to exceed the trigger level of the
FIFO, or the FIFO timer has expired (stale data), or
a single character has been received when the FIFO
is disabled.+0x02writeFIFO Control Register (FCR)
(This port does not exist on the 8250 and 16450
UART.)Bit 7Receiver Trigger Bit #1Bit 6Receiver Trigger Bit
#0These two bits control at what
point the receiver is to generate an interrupt
when the FIFO is active.76How many words are received
before an interrupt is generated0010141081114Bit 5Reserved, always 0.Bit 4Reserved, always 0.Bit 3DMA Mode Select. If Bit 0 is
set to "1" (FIFOs enabled), setting this bit changes
the operation of the -RXRDY and -TXRDY signals from
Mode 0 to Mode 1.Bit 2Transmit FIFO Reset. When a
"1" is written to this bit, the contents of the FIFO
are discarded. Any word currently being transmitted
will be sent intact. This function is useful in
aborting transfers.Bit 1Receiver FIFO Reset. When a
"1" is written to this bit, the contents of the FIFO
are discarded. Any word currently being assembled
in the shift register will be received
intact.Bit 016550 FIFO Enable. When set,
both the transmit and receive FIFOs are enabled.
Any contents in the holding register, shift
registers or FIFOs are lost when FIFOs are enabled
or disabled.+0x02readInterrupt Identification
RegisterBit 7FIFOs enabled. On the
8250/16450 UART, this bit is zero.Bit 6FIFOs enabled. On the
8250/16450 UART, this bit is zero.Bit 5Reserved, always 0.Bit 4Reserved, always 0.Bit 3Interrupt ID Bit #2. On the
8250/16450 UART, this bit is zero.Bit 2Interrupt ID Bit #1Bit 1Interrupt ID Bit #0.These
three bits combine to report the category of
event that caused the interrupt that is in
progress. These categories have priorities,
so if multiple categories of events occur at
the same time, the UART will report the more
important events first and the host must
resolve the events in the order they are
reported. All events that caused the current
interrupt must be resolved before any new
interrupts will be generated. (This is a
limitation of the PC architecture.)210PriorityDescription011FirstReceived Error (OE, PE, BI, or
FE)010SecondReceived Data Available110SecondTrigger level identification
(Stale data in receive buffer)001ThirdTransmitter has room for more
words (THRE)000FourthModem Status Change (-CTS, -DSR,
-RI, or -DCD)Bit 0Interrupt Pending Bit. If this
bit is set to "0", then at least one interrupt is
pending.+0x03write/readLine Control Register
(LCR)Bit 7Divisor Latch Access Bit
(DLAB). When set, access to the data
transmit/receive register (THR/RBR) and the
Interrupt Enable Register (IER) is disabled. Any
access to these ports is now redirected to the
Divisor Latch Registers. Setting this bit, loading
the Divisor Registers, and clearing DLAB should be
done with interrupts disabled.Bit 6Set Break. When set to "1",
the transmitter begins to transmit continuous
Spacing until this bit is set to "0". This
overrides any bits of characters that are being
transmitted.Bit 5Stick Parity. When parity is
enabled, setting this bit causes parity to always be
"1" or "0", based on the value of Bit 4.Bit 4Even Parity Select (EPS). When
parity is enabled and Bit 5 is "0", setting this bit
causes even parity to be transmitted and expected.
Otherwise, odd parity is used.Bit 3Parity Enable (PEN). When set
to "1", a parity bit is inserted between the last
bit of the data and the Stop Bit. The UART will
also expect parity to be present in the received
data.Bit 2Number of Stop Bits (STB). If
set to "1" and using 5-bit data words, 1.5 Stop Bits
are transmitted and expected in each data word. For
6, 7 and 8-bit data words, 2 Stop Bits are
transmitted and expected. When this bit is set to
"0", one Stop Bit is used on each data word.Bit 1Word Length Select Bit #1
(WLSB1)Bit 0Word Length Select Bit #0
(WLSB0)Together these
bits specify the number of bits in each data
word.10Word
Length005 Data
Bits016 Data
Bits107 Data
Bits118 Data
Bits+0x04write/readModem Control Register
(MCR)Bit 7Reserved, always 0.Bit 6Reserved, always 0.Bit 5Reserved, always 0.Bit 4Loop-Back Enable. When set to "1", the UART
transmitter and receiver are internally connected
together to allow diagnostic operations. In
addition, the UART modem control outputs are
connected to the UART modem control inputs. CTS is
connected to RTS, DTR is connected to DSR, OUT1 is
connected to RI, and OUT 2 is connected to
DCD.Bit 3OUT 2. An auxiliary output that the host
processor may set high or low. In the IBM PC serial
adapter (and most clones), OUT 2 is used to
tri-state (disable) the interrupt signal from the
8250/16450/16550 UART.Bit 2OUT 1. An auxiliary output that the host
processor may set high or low. This output is not
used on the IBM PC serial adapter.Bit 1Request to Send (RTS). When set to "1", the
output of the UART -RTS line is Low
(Active).Bit 0Data Terminal Ready (DTR). When set to "1",
the output of the UART -DTR line is Low
(Active).+0x05write/readLine Status Register
(LSR)Bit 7Error in Receiver FIFO. On the 8250/16450
UART, this bit is zero. This bit is set to "1" when
any of the bytes in the FIFO have one or more of the
following error conditions: PE, FE, or BI.Bit 6Transmitter Empty (TEMT). When set to "1",
there are no words remaining in the transmit FIFO
or the transmit shift register. The transmitter is
completely idle.Bit 5Transmitter Holding Register Empty (THRE).
When set to "1", the FIFO (or holding register) now
has room for at least one additional word to
transmit. The transmitter may still be transmitting
when this bit is set to "1".Bit 4Break Interrupt (BI). The receiver has
detected a Break signal.Bit 3Framing Error (FE). A Start Bit was detected
but the Stop Bit did not appear at the expected
time. The received word is probably
garbled.Bit 2Parity Error (PE). The parity bit was
incorrect for the word received.Bit 1Overrun Error (OE). A new word was received
and there was no room in the receive buffer. The
newly-arrived word in the shift register is
discarded. On 8250/16450 UARTs, the word in the
holding register is discarded and the newly- arrived
word is put in the holding register.Bit 0Data Ready (DR) One or more words are in the
receive FIFO that the host may read. A word must be
completely received and moved from the shift
register into the FIFO (or holding register for
8250/16450 designs) before this bit is set.+0x06write/readModem Status Register
(MSR)Bit 7Data Carrier Detect (DCD). Reflects the state
of the DCD line on the UART.Bit 6Ring Indicator (RI). Reflects the state of the
RI line on the UART.Bit 5Data Set Ready (DSR). Reflects the state of
the DSR line on the UART.Bit 4Clear To Send (CTS). Reflects the state of the
CTS line on the UART.Bit 3Delta Data Carrier Detect (DDCD). Set to "1"
if the -DCD line has changed state one more
time since the last time the MSR was read by the
host.Bit 2Trailing Edge Ring Indicator (TERI). Set to
"1" if the -RI line has had a low to high transition
since the last time the MSR was read by the
host.Bit 1Delta Data Set Ready (DDSR). Set to "1" if the
-DSR line has changed state one more time
since the last time the MSR was read by the
host.Bit 0Delta Clear To Send (DCTS). Set to "1" if the
-CTS line has changed state one more time
since the last time the MSR was read by the
host.+0x07write/readScratch Register (SCR). This register performs no
function in the UART. Any value can be written by the
host to this location and read by the host later
on.Beyond the 16550A UARTAlthough National Semiconductor has not offered any
components compatible with the 16550 that provide additional
features, various other vendors have. Some of these
components are described below. It should be understood
that to effectively utilize these improvements, drivers may
have to be provided by the chip vendor since most of the
popular operating systems do not support features beyond
those provided by the 16550.ST16650By default this part is similar to the NS16550A, but an
extended 32-byte send and receive buffer can be optionally
enabled. Made by StarTech.TIL16660By default this part behaves similar to the NS16550A,
but an extended 64-byte send and receive buffer can be
optionally enabled. Made by Texas Instruments.Hayes ESPThis proprietary plug-in card contains a 2048-byte send
and receive buffer, and supports data rates to
230.4Kbit/sec. Made by Hayes.In addition to these dumb UARTs, many vendors
produce intelligent serial communication boards. This type of
design usually provides a microprocessor that interfaces with
several UARTs, processes and buffers the data, and then alerts the
main PC processor when necessary. Because the UARTs are not
directly accessed by the PC processor in this type of
communication system, it is not necessary for the vendor to use
UARTs that are compatible with the 8250, 16450, or the 16550 UART.
This leaves the designer free to components that may have better
performance characteristics.Configuring the <filename>sio</filename> driverThe sio driver provides support
for NS8250-, NS16450-, NS16550 and NS16550A-based EIA RS-232C
(CCITT V.24) communications interfaces. Several multiport
cards are supported as well. See the &man.sio.4; manual page
for detailed technical documentation.Digi International (DigiBoard) PC/8Contributed by &a.awebster.email;. 26 August
1995.Here is a config snippet from a machine with a Digi
International PC/8 with 16550. It has 8 modems connected to
these 8 lines, and they work just great. Do not forget to
add options COM_MULTIPORT or it will not
work very well!device sio4 at isa? port 0x100 flags 0xb05
device sio5 at isa? port 0x108 flags 0xb05
device sio6 at isa? port 0x110 flags 0xb05
device sio7 at isa? port 0x118 flags 0xb05
device sio8 at isa? port 0x120 flags 0xb05
device sio9 at isa? port 0x128 flags 0xb05
device sio10 at isa? port 0x130 flags 0xb05
device sio11 at isa? port 0x138 flags 0xb05 irq 9The trick in setting this up is that the MSB of the
flags represent the last SIO port, in this case 11 so flags
are 0xb05.Boca 16Contributed by &a.whiteside.email;. 26 August
1995.The procedures to make a Boca 16 port board with FreeBSD
are pretty straightforward, but you will need a couple
things to make it work:You either need the kernel sources installed so you
can recompile the necessary options or you will need
someone else to compile it for you. The 2.0.5 default
kernel does not come with
multiport support enabled and you will need to add a
device entry for each port anyways.Two, you will need to know the interrupt and IO
setting for your Boca Board so you can set these options
properly in the kernel.One important note — the actual UART chips for the
Boca 16 are in the connector box, not on the internal board
itself. So if you have it unplugged, probes of those ports
will fail. I have never tested booting with the box
unplugged and plugging it back in, and I suggest you do not
either.If you do not already have a custom kernel
configuration file set up, refer to Kernel
Configuration chapter of the FreeBSD Handbook for
general procedures. The following are the specifics for the
Boca 16 board and assume you are using the kernel name
MYKERNEL and editing with vi.Add the line
options COM_MULTIPORT
to the config file.Where the current device
sion lines are, you
will need to add 16 more devices. The
following example is for a Boca Board with an interrupt
of 3, and a base IO address 100h. The IO address for
Each port is +8 hexadecimal from the previous port, thus
the 100h, 108h, 110h... addresses.device sio1 at isa? port 0x100 flags 0x1005
device sio2 at isa? port 0x108 flags 0x1005
device sio3 at isa? port 0x110 flags 0x1005
device sio4 at isa? port 0x118 flags 0x1005
…
device sio15 at isa? port 0x170 flags 0x1005
device sio16 at isa? port 0x178 flags 0x1005 irq 3The flags entry must be changed
from this example unless you are using the exact same
sio assignments. Flags are set according to
0xMYY
where M indicates the minor
number of the master port (the last port on a Boca 16)
and YY indicates if FIFO is
enabled or disabled(enabled), IRQ sharing is used(yes)
and if there is an AST/4 compatible IRQ control
register(no). In this example, flags
0x1005 indicates that the master port
is sio16. If I added another board and assigned sio17
through sio28, the flags for all 16 ports on
that board would be 0x1C05, where
1C indicates the minor number of the master port. Do
not change the 05 setting.Save and complete the kernel configuration,
recompile, install and reboot. Presuming you have
successfully installed the recompiled kernel and have it
set to the correct address and IRQ, your boot message
should indicate the successful probe of the Boca ports
as follows: (obviously the sio numbers, IO and IRQ could
be different)sio1 at 0x100-0x107 flags 0x1005 on isa
sio1: type 16550A (multiport)
sio2 at 0x108-0x10f flags 0x1005 on isa
sio2: type 16550A (multiport)
sio3 at 0x110-0x117 flags 0x1005 on isa
sio3: type 16550A (multiport)
sio4 at 0x118-0x11f flags 0x1005 on isa
sio4: type 16550A (multiport)
sio5 at 0x120-0x127 flags 0x1005 on isa
sio5: type 16550A (multiport)
sio6 at 0x128-0x12f flags 0x1005 on isa
sio6: type 16550A (multiport)
sio7 at 0x130-0x137 flags 0x1005 on isa
sio7: type 16550A (multiport)
sio8 at 0x138-0x13f flags 0x1005 on isa
sio8: type 16550A (multiport)
sio9 at 0x140-0x147 flags 0x1005 on isa
sio9: type 16550A (multiport)
sio10 at 0x148-0x14f flags 0x1005 on isa
sio10: type 16550A (multiport)
sio11 at 0x150-0x157 flags 0x1005 on isa
sio11: type 16550A (multiport)
sio12 at 0x158-0x15f flags 0x1005 on isa
sio12: type 16550A (multiport)
sio13 at 0x160-0x167 flags 0x1005 on isa
sio13: type 16550A (multiport)
sio14 at 0x168-0x16f flags 0x1005 on isa
sio14: type 16550A (multiport)
sio15 at 0x170-0x177 flags 0x1005 on isa
sio15: type 16550A (multiport)
sio16 at 0x178-0x17f irq 3 flags 0x1005 on isa
sio16: type 16550A (multiport master)If the messages go by too fast to see,
&prompt.root; dmesg | more
will show you the boot messages.Next, appropriate entries in
/dev for the devices must be made
using the /dev/MAKEDEV
script. This step can be omitted if you are running
FreeBSD 5.X with a kernel that has &man.devfs.5;
support compiled in.If you do need to create the /dev
entries, run the following as root:&prompt.root; cd /dev
&prompt.root; ./MAKEDEV tty1
&prompt.root; ./MAKEDEV cua1(everything in between)
&prompt.root; ./MAKEDEV ttyg
&prompt.root; ./MAKEDEV cuagIf you do not want or need call-out devices for some
reason, you can dispense with making the
cua* devices.If you want a quick and sloppy way to make sure the
devices are working, you can simply plug a modem into
each port and (as root)
&prompt.root; echo at > ttyd*
for each device you have made. You
should see the RX lights flash for each
working port.Support for Cheap Multi-UART CardsContributed by Helge Oldach
hmo@sep.hamburg.com, September
1999Ever wondered about FreeBSD support for your 20$
multi-I/O card with two (or more) COM ports, sharing IRQs?
Here is how:Usually the only option to support these kind of boards
is to use a distinct IRQ for each port. For example, if
your CPU board has an on-board COM1
port (aka sio0–I/O address
0x3F8 and IRQ 4) and you have an extension board with two
UARTs, you will commonly need to configure them as
COM2 (aka
sio1–I/O address 0x2F8 and
IRQ 3), and the third port (aka
sio2) as I/O 0x3E8 and IRQ 5.
Obviously this is a waste of IRQ resources, as it should be
basically possible to run both extension board ports using a
single IRQ with the COM_MULTIPORT
configuration described in the previous sections.Such cheap I/O boards commonly have a 4 by 3 jumper
matrix for the COM ports, similar to the following: o o o *
Port A |
o * o *
Port B |
o * o o
IRQ 2 3 4 5Shown here is port A wired for IRQ 5 and port B wired
for IRQ 3. The IRQ columns on your specific board may
vary—other boards may supply jumpers for IRQs 3, 4, 5,
and 7 instead.One could conclude that wiring both ports for IRQ 3
using a handcrafted wire-made jumper covering all three
connection points in the IRQ 3 column would solve the issue,
but no. You cannot duplicate IRQ 3 because the output
drivers of each UART are wired in a totem
pole fashion, so if one of the UARTs drives IRQ 3,
the output signal will not be what you would expect.
Depending on the implementation of the extension board or
your motherboard, the IRQ 3 line will continuously stay up,
or always stay low.You need to decouple the IRQ drivers for the two UARTs,
so that the IRQ line of the board only goes up if (and only
if) one of the UARTs asserts a IRQ, and stays low otherwise.
The solution was proposed by Joerg Wunsch
j@ida.interface-business.de: To solder up a
wired-or consisting of two diodes (Germanium or
Schottky-types strongly preferred) and a 1 kOhm resistor.
Here is the schematic, starting from the 4 by 3 jumper field
above: Diode
+---------->|-------+
/ |
o * o o | 1 kOhm
Port A +----|######|-------+
o * o o | |
Port B `-------------------+ ==+==
o * o o | Ground
\ |
+--------->|-------+
IRQ 2 3 4 5 DiodeThe cathodes of the diodes are connected to a common
point, together with a 1 kOhm pull-down resistor. It is
essential to connect the resistor to ground to avoid
floating of the IRQ line on the bus.Now we are ready to configure a kernel. Staying with
this example, we would configure:# standard on-board COM1 port
device sio0 at isa? port "IO_COM1" flags 0x10
# patched-up multi-I/O extension board
options COM_MULTIPORT
device sio1 at isa? port "IO_COM2" flags 0x205
device sio2 at isa? port "IO_COM3" flags 0x205 irq 3Note that the flags setting for
sio1 and
sio2 is truly essential; refer to
&man.sio.4; for details. (Generally, the
2 in the "flags" attribute refers to
sio2 which holds the IRQ, and you
surely want a 5 low nibble.) With kernel
verbose mode turned on this should yield something similar
to this:sio0: irq maps: 0x1 0x11 0x1 0x1
sio0 at 0x3f8-0x3ff irq 4 flags 0x10 on isa
sio0: type 16550A
sio1: irq maps: 0x1 0x9 0x1 0x1
sio1 at 0x2f8-0x2ff flags 0x205 on isa
sio1: type 16550A (multiport)
sio2: irq maps: 0x1 0x9 0x1 0x1
sio2 at 0x3e8-0x3ef irq 3 flags 0x205 on isa
sio2: type 16550A (multiport master)Though /sys/i386/isa/sio.c is
somewhat cryptic with its use of the irq maps
array above, the basic idea is that you observe
0x1 in the first, third, and fourth
place. This means that the corresponding IRQ was set upon
output and cleared after, which is just what we would
expect. If your kernel does not display this behavior, most
likely there is something wrong with your wiring.Configuring the <filename>cy</filename> driverContributed by Alex Nash. 6 June
1996.The Cyclades multiport cards are based on the
cy driver instead of the usual
sio driver used by other multiport
cards. Configuration is a simple matter of:Add the cy device to your
kernel configuration (note that your irq and iomem
settings may differ).device cy0 at isa? irq 10 iomem 0xd4000 iosiz 0x2000Rebuild and install the new kernel.Make the device nodes by typing (the following
example assumes an 8-port board)You can omit this part if you are running FreeBSD 5.X
with &man.devfs.5;.:&prompt.root; cd /dev
&prompt.root; for i in 0 1 2 3 4 5 6 7;do ./MAKEDEV cuac$i ttyc$i;doneIf appropriate, add dialup entries to
/etc/ttys by duplicating serial
device (ttyd) entries and using
ttyc in place of
ttyd. For example:ttyc0 "/usr/libexec/getty std.38400" unknown on insecure
ttyc1 "/usr/libexec/getty std.38400" unknown on insecure
ttyc2 "/usr/libexec/getty std.38400" unknown on insecure
…
ttyc7 "/usr/libexec/getty std.38400" unknown on insecureReboot with the new kernel.Configuring the <filename>si</filename> driverContributed by &a.nsayer.email;. 25 March
1998.The Specialix SI/XIO and SX multiport cards use the
si driver. A single machine can have
up to 4 host cards. The following host cards are
supported:ISA SI/XIO host card (2 versions)EISA SI/XIO host cardPCI SI/XIO host cardISA SX host cardPCI SX host cardAlthough the SX and SI/XIO host cards look markedly
different, their functionality are basically the same. The
host cards do not use I/O locations, but instead require a 32K
chunk of memory. The factory configuration for ISA cards
places this at 0xd0000-0xd7fff. They also
require an IRQ. PCI cards will, of course, auto-configure
themselves.You can attach up to 4 external modules to each host
card. The external modules contain either 4 or 8 serial
ports. They come in the following varieties:SI 4 or 8 port modules. Up to 57600 bps on each port
supported.XIO 8 port modules. Up to 115200 bps on each port
supported. One type of XIO module has 7 serial and 1 parallel
port.SXDC 8 port modules. Up to 921600 bps on each port
supported. Like XIO, a module is available with one parallel
port as well.To configure an ISA host card, add the following line to
your kernel configuration file, changing the numbers as
appropriate:device si0 at isa? iomem 0xd0000 irq 11Valid IRQ numbers are 9, 10, 11, 12 and 15 for SX ISA host
cards and 11, 12 and 15 for SI/XIO ISA host cards.To configure an EISA or PCI host card, use this line:device si0After adding the configuration entry, rebuild and
install your new kernel.The following step, is not necessary if you are using
&man.devfs.5; in FreeBSD 5.X.After rebooting with the new kernel, you need to make the
device nodes in /dev. The MAKEDEV script
will take care of this for you. Count how many total ports
you have and type:&prompt.root; cd /dev
&prompt.root; ./MAKEDEV ttyAnn cuaAnn(where nn is the number of
ports)If you want login prompts to appear on these ports, you
will need to add lines like this to
/etc/ttys:ttyA01 "/usr/libexec/getty std.9600" vt100 on insecureChange the terminal type as appropriate. For modems,
dialup or
unknown is fine.