Copyright © 1996 &a.uhclem;, All Rights Reserved. Synchronous 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 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 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.
In 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 asserting or deasserting one more more of these extra signals.
The function of each of these additional signals is defined in
the EIA RS232-C standard.
In 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.
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 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.
Note that 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.
The 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 translater 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.
The 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.
Baud is a measurment 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.
Starting 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.
There have been several versions and subsequent generations of
the INS8250 UART. Each major version is described below.
National 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.
The 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:
Over 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 suze 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 Microsoft
MSD 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.
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.
* To 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 erattas that describe bugs in the A, B and C
revisions of the parts, so this bias in COMTEST must be taken into
account.
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:
The 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.
A description of the I/O ports of the 8250/16450/16550 UART is
provided below.
Although 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.
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.
UART Universal Asynchronous Receiver/Transmitter
USART Universal Synchronous-Asynchronous Receiver/Transmitter
Error (6)...Timeout interrupt failed: IIR = c1 LSR = 61