TCP/IP WITH NO TNC

                        By: Mike Hooper, KF6PU

  TNCs run in the kissmode and SCC cards such as the DRSI PCPA have long
been the staple of most TCP/IPers running NOS and Net versions of the
KA9Q Internet Suite of Protocols. There is a much less expensive and
more elegant solution for a 1200 baud hardware interface.

 "Baycom" style modems have enjoyed international popularity for some
time now among those running packet software that employs the "software
tnc" approach. Baycom, PMP, and Eskay Packet (SP) V6.10 with the TFPCX
driver are very popular.

 Most Baycom style modems are designed around either the AMD 7910 "World
Chip" IC , Texas Instruments TCM 3105, or the EXAR 2206/2211 pair. Most
commercial TNCs use these chips for the modem section. For example, the
AEA PK-88 uses the AMD 7910, The Kantronics KAM and KPC-4 use the TCM
3105 for VHF-UHF ports and the MFJ 1270/1274 use the EXAR pair. Few
external parts are required for these modems and they are easy to
construct. The TCM 3105 modem can be built for under $25.00 and can be
built so small as to fit into a DB25 shell. 

 Thanks to the efforts of Pawel Jalocha, SP9VRC (SP9VRC@SP9ZDN.POL.EU)
(jalocha@chopin.ifj.edu.pl) a packet driver conforming (to a sufficient
extent) to the "FTP packet driver specification" has been developed that
serves as an interface between application software (e.g. KA9Q NOS) and
a modem connected to the RS232 port (e.g. Baycom modem). This driver
along with documentation and accessory files have been distributed in
this country under the filename "AX25DRV.ZIP" .  Be sure to use the
January 4, 1993 version as prior releases have bugs.

 Jawel's packet driver enables the use of unsquelched audio as the
driver is able to deliver "channel busy" status by analyzing incoming
data.

 The driver is loaded into memory before NOS is run. The utility
"TERMIN.COM" is used to remove the driver from memory when one exits
from NOS. Below is a batch file that sets up the driver for use on COM2
with NOS :
          @echo off
          c:
          cd\net
          ax25 -b1200 -B2f8 -I3p -cd -h350
          nos.exe
          termin.com
          cls

 The switches invoked with ax25.com specify: 1200 baud, COM2, IRQ
priority (given to IRQ 3 in this example), carrier detect option, and
txdelay. Slottime and persist default to 100 msec. and 64 but are
adjustable with the -s and -p switches. The software interrupt defaults
to 0x60 but is adjustable from 60 through 63 with the -i switch. Carrier
detect threshold and TXtail are also adjustable with the -T and -t
switches. A switch need not be specified if the default value is used.

 Within AUTOEXEC.NOS is the following attach statement (your version of
NOS must be compiled with the attach packet hardware interface option):

          attach packet 0x60 144 5 256


 I have been using this driver with both the WG7J and KB7YW versions of
NOS for several weeks. My system utilizes three hardware interfaces two
of which are handled by a DRSI card. During this period I have had the
opportunity to use a KAM (v6.0), PK-88, and a PK-232 with the TAPR DCD
mod and in no case have I found these superior to a TCM 3105 modem used
with Jawel's ax25 driver. The driver does not work well on an XT clone.
My computer is a 386-33 SX using a pair of 16550 AFNs. I find it useful
to reset the computer before running NOS to "clear" the comports.

 Work is currently in progress on adapting the G3RUH 9600 modem for use
with the AX.25 driver. Potential problems concerning interrupt latency
issues may complicate matters. If success is achieved it would
substantially reduce the cost of running TCP/IP at 9600 baud.        

 -73-
kf6pu@wb6ymh.#soca.usa