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phoenix |
LINUX DRIVER FOR SOUNDCARDS AS AX.25 MODEMS
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Thomas M. Sailer, HB9JNX/AE4WA,
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This driver allows either SoundBlaster (sbc) or Windows Sound System (wss)
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compatible soundcards to be used as either 1200 baud AFSK or 9600 baud FSK
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AX.25 packet radio modems. Only half duplex operation is supported; an
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attempt to include full duplex support failed because the hardware did
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not support it (it appeared that the card only provides one DMA channel,
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although the codec chip would support two channels). The driver needs
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some processing power! A 66 MHz 486 DX2 is a minimum requirement. Otherwise
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interactive performance of the computer may become sluggish. This driver
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does *not* support telephone modem standards, it is intended for radio
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use only.
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The Interface of the driver
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The driver provides kernel network drivers named sm[0-3]. sethdlc
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from the ax25 utilities may be used to set driver states etc. Users
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of userland AX.25 stacks may use the net2kiss utility (also available
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in the ax25 utilities package) to convert packets of a network interface
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to a KISS stream on a pseudo tty. There's also a patch available from
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me for WAMPES which allows attaching a kernel network interface directly.
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Configuring the driver
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Some sound cards need to be initialized before they operate in either
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SoundBlaster or WSS compatibility mode. The driver does _NOT_ do this;
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you may use the standard linux sound driver to initialize the soundcard;
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compile it as a module, and do
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insmod sound
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rmmod sound
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The soundcard should then be initialized correctly. If this does not help,
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you'll have to write your own initialization utility.
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Every time the driver is inserted into the kernel, it has to know which
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modems it should access at which ports. This can be done with the setbaycom
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utility. If you are only using one modem, you can also configure the
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driver from the insmod command line (or by means of an option line in
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/etc/modules.conf).
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Examples:
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insmod soundmodem mode="sbc:afsk1200" iobase=0x220 irq=5 dma=1
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sethdlc -i sm0 -p mode "sbc:afsk1200" io 0x220 irq 5 dma 1
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Both lines configure the first port to drive a soundblaster card
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in 1200 baud AFSK mode.
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The channel access parameters can be set with sethdlc -a or kissparms.
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Note that both utilities interpret the values slightly different.
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Input and output levels
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It is important that the input and output levels are adjusted properly.
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There are two utilities, available in the ax25 utilities distribution,
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to facilitate this: smmixer and smdiag. smdiag allows you to display
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the input signal in an oscilloscope like display or an eye diagram.
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smmixer allows you to adjust input/output levels. See the respective
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man pages.
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Transmitter keying
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Since soundcards do not have a DC coupled output; PTT keying options include
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the following:
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* VOX circuitry
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* Serial port pin
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* Parallel port pin
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* MPU401 MIDI output via a retriggerable monoflop.
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Circuit schematics may be found at
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http://www.ife.ee.ethz.ch/~sailer/pcf/ptt_circ/ptt.html.
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Compatibility with the rest of the Linux kernel
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The sound driver and the soundcard modem driver compete for the same
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hardware resources. Of course only one driver can access a given
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interface at a time. Worse yet, the sound driver grabs the soundcard
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at startup time. Therefore the soundcard modem driver subsequently won't
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be able to access the soundcard. You might therefore find it necessary to
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unload the sound driver before using the soundcard modem driver.
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vy 73s de
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Tom Sailer, sailer@ife.ee.ethz.ch
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hb9jnx @ hb9w.ampr.org
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