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Using cheap serial multi-port cards on FreeBSD

Note: Before you continue reading please make sure that you have read and understood the disclaimer.

Ever 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  5

Shown 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    Diode

The 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 3

Note that the flags setting for sio1 and sio2 is truly essential; refer to 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.

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