LINUX DRIVER FOR SOUNDCARDS AS AX.25 MODEMS

       Thomas M. Sailer, HB9JNX/AE4WA, <sailer@ife.ee.ethz.ch>

This driver allows either SoundBlaster (sbc) or Windows Sound System (wss)
compatible soundcards to be used as either 1200 baud AFSK or 9600 baud FSK
AX.25 packet radio modems. Only half duplex operation is supported; an
attempt to include full duplex support failed because the hardware did
not support it (it appeared that the card only provides one DMA channel,
although the codec chip would support two channels). The driver needs
some processing power! A 66 MHz 486 DX2 is a minimum requirement.  Otherwise
interactive performance of the computer may become sluggish. This driver
does *not* support telephone modem standards, it is intended for radio
use only.


The Interface of the driver

The driver provides kernel network drivers named sm[0-3]. sethdlc 
from the ax25 utilities may be used to set driver states etc. Users
of userland AX.25 stacks may use the net2kiss utility (also available
in the ax25 utilities package) to convert packets of a network interface
to a KISS stream on a pseudo tty. There's also a patch available from
me for WAMPES which allows attaching a kernel network interface directly.


Configuring the driver

Some sound cards need to be initialized before they operate in either
SoundBlaster or WSS compatibility mode. The driver does _NOT_ do this;
you may use the standard linux sound driver to initialize the soundcard;
compile it as a module, and do
  insmod sound
  rmmod sound
The soundcard should then be initialized correctly. If this does not help,
you'll have to write your own initialization utility.

Every time the driver is inserted into the kernel, it has to know which
modems it should access at which ports. This can be done with the setbaycom
utility. If you are only using one modem, you can also configure the
driver from the insmod command line (or by means of an option line in
/etc/conf.modules).

Examples:
  insmod soundmodem mode="sbc:afsk1200" iobase=0x220 irq=5 dma=1
  sethdlc -i sm0 -p mode "sbc:afsk1200" io 0x220 irq 5 dma 1

Both lines configure the first port to drive a soundblaster card
in 1200 baud AFSK mode.

The channel access parameters can be set with sethdlc -a or kissparms.
Note that both utilities interpret the values slightly different.


Input and output levels

It is important that the input and output levels are adjusted properly.
There are two utilities, available in the ax25 utilities distribution,
to facilitate this: smmixer and smdiag. smdiag allows you to display
the input signal in an oscilloscope like display or an eye diagram.
smmixer allows you to adjust input/output levels. See the respective
man pages.


Transmitter keying

Since soundcards do not have a DC coupled output; PTT keying options include
the following:
* VOX circuitry
* Serial port pin
* Parallel port pin
* MPU401 MIDI output via a retriggerable monoflop.
Circuit schematics may be found at
http://www.ife.ee.ethz.ch/~sailer/pcf/ptt_circ/ptt.html.


Compatibility with the rest of the Linux kernel

The sound driver and the soundcard modem driver compete for the same
hardware resources. Of course only one driver can access a given
interface at a time. Worse yet, the sound driver grabs the soundcard
at startup time. Therefore the soundcard modem driver subsequently won't
be able to access the soundcard. You might therefore find it necessary to
unload the sound driver before using the soundcard modem driver.



vy 73s de
Tom Sailer, sailer@ife.ee.ethz.ch
hb9jnx @ hb9w.ampr.org