New section on using cheap multiport UARTS.
Submitted by: Helge Oldach <hmo@sep.hamburg.com>
This commit is contained in:
Nik Clayton
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2 changed files with 226 additions and 2 deletions
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@ -1,7 +1,7 @@
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<!--
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<!--
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The FreeBSD Documentation Project
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The FreeBSD Documentation Project
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$FreeBSD$
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$FreeBSD: doc/en_US.ISO_8859-1/books/handbook/hw/chapter.sgml,v 1.26 1999/09/06 06:52:57 peter Exp $
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-->
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-->
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<chapter id="hw">
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<chapter id="hw">
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@ -2786,6 +2786,118 @@ sio16: type 16550A (multiport master)</screen>
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</step>
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</step>
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</procedure>
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</procedure>
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</sect4>
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</sect4>
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<sect4>
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<title>Support for Cheap Multi-UART Cards</title>
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<para><emphasis>Contributed by Helge Oldach
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<email>hmo@sep.hamburg.com</email>, September
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1999</emphasis></para>
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<para>Ever wondered about FreeBSD support for your 20$ multi-I/O
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card with two (or more) COM ports, sharing IRQs? Here's
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how:</para>
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<para>Usually the only option to support these kind of boards is to
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use a distinct IRQ for each port. For example, if your CPU board
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has an on-board <devicename>COM1</devicename> port (aka
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<devicename>sio0</devicename>–I/O address 0x3F8 and IRQ 4)
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and you have an extension board with two UARTs, you will commonly
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need to configure them as <devicename>COM2</devicename> (aka
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<devicename>sio1</devicename>–I/O address 0x2F8 and IRQ 3),
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and the third port (aka <devicename>sio2</devicename>) as I/O
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0x3E8 and IRQ 5. Obviously this is a waste of IRQ ressources, as
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it should be basically possible to run both extension board ports
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using a single IRQ with the <literal>COM_MULTIPORT</literal>
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configuration described in the previous sections.</para>
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<para>Such cheap I/O boards commonly have a 4 by 3 jumper matrix for
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the COM ports, similar to the following:</para>
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<programlisting> o o o *
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Port A |
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o * o *
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Port B |
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o * o o
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IRQ 2 3 4 5</programlisting>
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<para>Shown here is port A wired for IRQ 5 and port B wired for IRQ
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3. The IRQ columns on your specific board may vary—other
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boards may supply jumpers for IRQs 3, 4, 5, and 7 instead.</para>
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<para>One could conclude that wiring both ports for IRQ 3 using a
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handcrafted wire-made jumper covering all three connection points
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in the IRQ 3 column would solve the issue, but no. You cannot
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duplicate IRQ 3 because the output drivers of each UART are wired
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in a “totem pole” fashion, so if one of the UARTs
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drives IRQ 3, the output signal will not be what you would expect.
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Depending on the implementation of the extension board or your
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motherboard, the IRQ 3 line will continuously stay up, or always
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stay low.</para>
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<para>You need to decouple the IRQ drivers for the two UARTs, so
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that the IRQ line of the board only goes up if (and only if) one
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of the UARTs asserts a IRQ, and stays low otherwise. The solution
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was proposed by Jrg Wunsch
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<email>j@ida.interface-business.de</email>: To solder up a
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wired-or consisting of two diodes (Germanium or Schottky-types
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strongly preferred) and a 1 kOhm resistor. Here is the schematic,
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starting from the 4 by 3 jumper field above:</para>
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<programlisting> Diode
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+---------->|-------+
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/ |
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o * o o | 1 kOhm
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Port A +----|######|-------+
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o * o o | |
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Port B `-------------------+ ==+==
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o * o o | Ground
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\ |
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+--------->|-------+
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IRQ 2 3 4 5 Diode</programlisting>
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<para>The cathodes of the diodes are connected to a common point,
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together with a 1 kOhm pull-down resistor. It is essential to
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connect the resistor to ground to avoid floating of the IRQ line
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on the bus.</para>
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<para>Now we are ready to configure a kernel. Staying with this
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example, we would configure:</para>
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<programlisting># standard on-board COM1 port
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device sio0 at isa? port "IO_COM1" tty flags 0x10
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# patched-up multi-I/O extension board
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options COM_MULTIPORT
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device sio1 at isa? port "IO_COM2" tty flags 0x205
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device sio2 at isa? port "IO_COM3" tty flags 0x205 irq 3</programlisting>
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<para>Note that the <literal>flags</literal> setting for
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<devicename>sio1</devicename> and <devicename>sio2</devicename> is
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truely essential; refer to
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&man.sio.4; for details. (Generally, the <literal>2</literal> in
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the "flags" attribute refers to <devicename>sio</devicename>2
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which holds the IRQ, and you surely want a <literal>5</literal>
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low nibble.) With kernel verbose mode turned on this should yield
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something similar to this:</para>
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<screen>sio0: irq maps: 0x1 0x11 0x1 0x1
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sio0 at 0x3f8-0x3ff irq 4 flags 0x10 on isa
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sio0: type 16550A
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sio1: irq maps: 0x1 0x9 0x1 0x1
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sio1 at 0x2f8-0x2ff flags 0x205 on isa
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sio1: type 16550A (multiport)
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sio2: irq maps: 0x1 0x9 0x1 0x1
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sio2 at 0x3e8-0x3ef irq 3 flags 0x205 on isa
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sio2: type 16550A (multiport master)</screen>
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<para>Though <filename>/sys/i386/isa/sio.c</filename> is somewhat
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cryptic with its use of the “irq maps” array above,
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the basic idea is that you observe <literal>0x1</literal> in the
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first, third, and fourth place. This means that the corresponding
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IRQ was set upon output and cleared after, which is just what we
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would expect. If your kernel does not display this behaviour, most
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likely there is something wrong with your wiring.</para>
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</sect4>
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</sect3>
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</sect3>
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<sect3 id="cy">
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<sect3 id="cy">
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|
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@ -1,7 +1,7 @@
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||||||
<!--
|
<!--
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||||||
The FreeBSD Documentation Project
|
The FreeBSD Documentation Project
|
||||||
|
|
||||||
$FreeBSD$
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$FreeBSD: doc/en_US.ISO_8859-1/books/handbook/hw/chapter.sgml,v 1.26 1999/09/06 06:52:57 peter Exp $
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||||||
-->
|
-->
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||||||
|
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||||||
<chapter id="hw">
|
<chapter id="hw">
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||||||
|
|
@ -2786,6 +2786,118 @@ sio16: type 16550A (multiport master)</screen>
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||||||
</step>
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</step>
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||||||
</procedure>
|
</procedure>
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||||||
</sect4>
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</sect4>
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<sect4>
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<title>Support for Cheap Multi-UART Cards</title>
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<para><emphasis>Contributed by Helge Oldach
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<email>hmo@sep.hamburg.com</email>, September
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1999</emphasis></para>
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||||||
|
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||||||
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<para>Ever wondered about FreeBSD support for your 20$ multi-I/O
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||||||
|
card with two (or more) COM ports, sharing IRQs? Here's
|
||||||
|
how:</para>
|
||||||
|
|
||||||
|
<para>Usually the only option to support these kind of boards is to
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||||||
|
use a distinct IRQ for each port. For example, if your CPU board
|
||||||
|
has an on-board <devicename>COM1</devicename> port (aka
|
||||||
|
<devicename>sio0</devicename>–I/O address 0x3F8 and IRQ 4)
|
||||||
|
and you have an extension board with two UARTs, you will commonly
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||||||
|
need to configure them as <devicename>COM2</devicename> (aka
|
||||||
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<devicename>sio1</devicename>–I/O address 0x2F8 and IRQ 3),
|
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and the third port (aka <devicename>sio2</devicename>) as I/O
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||||||
|
0x3E8 and IRQ 5. Obviously this is a waste of IRQ ressources, as
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||||||
|
it should be basically possible to run both extension board ports
|
||||||
|
using a single IRQ with the <literal>COM_MULTIPORT</literal>
|
||||||
|
configuration described in the previous sections.</para>
|
||||||
|
|
||||||
|
<para>Such cheap I/O boards commonly have a 4 by 3 jumper matrix for
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||||||
|
the COM ports, similar to the following:</para>
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|
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<programlisting> o o o *
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Port A |
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o * o *
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Port B |
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o * o o
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IRQ 2 3 4 5</programlisting>
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<para>Shown here is port A wired for IRQ 5 and port B wired for IRQ
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3. The IRQ columns on your specific board may vary—other
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||||||
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boards may supply jumpers for IRQs 3, 4, 5, and 7 instead.</para>
|
||||||
|
|
||||||
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<para>One could conclude that wiring both ports for IRQ 3 using a
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||||||
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handcrafted wire-made jumper covering all three connection points
|
||||||
|
in the IRQ 3 column would solve the issue, but no. You cannot
|
||||||
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duplicate IRQ 3 because the output drivers of each UART are wired
|
||||||
|
in a “totem pole” fashion, so if one of the UARTs
|
||||||
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drives IRQ 3, the output signal will not be what you would expect.
|
||||||
|
Depending on the implementation of the extension board or your
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||||||
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motherboard, the IRQ 3 line will continuously stay up, or always
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||||||
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stay low.</para>
|
||||||
|
|
||||||
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<para>You need to decouple the IRQ drivers for the two UARTs, so
|
||||||
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that the IRQ line of the board only goes up if (and only if) one
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||||||
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of the UARTs asserts a IRQ, and stays low otherwise. The solution
|
||||||
|
was proposed by Jrg Wunsch
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||||||
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<email>j@ida.interface-business.de</email>: To solder up a
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||||||
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wired-or consisting of two diodes (Germanium or Schottky-types
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||||||
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strongly preferred) and a 1 kOhm resistor. Here is the schematic,
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starting from the 4 by 3 jumper field above:</para>
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<programlisting> Diode
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+---------->|-------+
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/ |
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o * o o | 1 kOhm
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Port A +----|######|-------+
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o * o o | |
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Port B `-------------------+ ==+==
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o * o o | Ground
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\ |
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+--------->|-------+
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IRQ 2 3 4 5 Diode</programlisting>
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<para>The cathodes of the diodes are connected to a common point,
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together with a 1 kOhm pull-down resistor. It is essential to
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connect the resistor to ground to avoid floating of the IRQ line
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on the bus.</para>
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||||||
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||||||
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<para>Now we are ready to configure a kernel. Staying with this
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example, we would configure:</para>
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||||||
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<programlisting># standard on-board COM1 port
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||||||
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device sio0 at isa? port "IO_COM1" tty flags 0x10
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||||||
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# patched-up multi-I/O extension board
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||||||
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options COM_MULTIPORT
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||||||
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device sio1 at isa? port "IO_COM2" tty flags 0x205
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device sio2 at isa? port "IO_COM3" tty flags 0x205 irq 3</programlisting>
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||||||
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||||||
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<para>Note that the <literal>flags</literal> setting for
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||||||
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<devicename>sio1</devicename> and <devicename>sio2</devicename> is
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||||||
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truely essential; refer to
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||||||
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&man.sio.4; for details. (Generally, the <literal>2</literal> in
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||||||
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the "flags" attribute refers to <devicename>sio</devicename>2
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||||||
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which holds the IRQ, and you surely want a <literal>5</literal>
|
||||||
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low nibble.) With kernel verbose mode turned on this should yield
|
||||||
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something similar to this:</para>
|
||||||
|
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||||||
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<screen>sio0: irq maps: 0x1 0x11 0x1 0x1
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sio0 at 0x3f8-0x3ff irq 4 flags 0x10 on isa
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sio0: type 16550A
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sio1: irq maps: 0x1 0x9 0x1 0x1
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sio1 at 0x2f8-0x2ff flags 0x205 on isa
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sio1: type 16550A (multiport)
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sio2: irq maps: 0x1 0x9 0x1 0x1
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sio2 at 0x3e8-0x3ef irq 3 flags 0x205 on isa
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sio2: type 16550A (multiport master)</screen>
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||||||
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||||||
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<para>Though <filename>/sys/i386/isa/sio.c</filename> is somewhat
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||||||
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cryptic with its use of the “irq maps” array above,
|
||||||
|
the basic idea is that you observe <literal>0x1</literal> in the
|
||||||
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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 behaviour, most
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||||||
|
likely there is something wrong with your wiring.</para>
|
||||||
|
</sect4>
|
||||||
</sect3>
|
</sect3>
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||||||
|
|
||||||
<sect3 id="cy">
|
<sect3 id="cy">
|
||||||
|
|
|
||||||
Loading…
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