
           ***   USING TWO TNC'S WITH ONE RADIO  ***
                     BY: Jeff King WB8WKA

On occasion, one has the need to use two TNC's on the same frequency. Examples
of this could include a NETROM and normal TNC, the TEXNET  conference software,
KA9Q's TCP/IP package with KISS software etc etc... Frequently, one will want
to run one of these packages while still running a "normal" AX.25 tnc on
frequency. While one could run two radios on two antennas on the same
frequency, this would be quite wasteful of resources.

While you could simply parallel the two tnc's to the radio in "OR" fashion you
could still have problems with the TNC's trashing each other's packets when
both decided to transmit at the same time. What is needed is a way for each TNC
to know when the other is transmitting.

In the TNC2, TAPR (Tucson Amateur Packet Radio) provided a RF-DCD pin. While
this was intended for use on a shared voice/packet frequency, it comes in quite
handy for our purposes. To allow each respective TNC to "see" the other when
the other is transmitting, we simply cross connect this RF-DCD pin (5) with the
PTT pin (3) of the opposite TNC. The TNC's PTT's are isolated from each other
by a diode OR gate. Both the speaker (RX audio) and mic audio (TX audio) are
mixed by a resistor mixer. The values may need to be juggled for each radio but
the values don't seem to be very critical. If you don't have a speaker
connected, be sure to terminate each radio with a 8-30 ohm resistor. Below is a
diagram of the layout for the circuit:


                   TWO TNC's to ONE radio 

     TNC  A                                                       TNC B

  Use      Pin #                                                Pin #   Use

TX Audio    1 ___/\/\/\_____________            _______/\/\/\____ 1    TX Audio
                 220 Ohm           |            |      220 Ohm
                                   |            |
Ground      2 ___                  |            |             ___ 2    Ground
                |                  |            |             |
                ^                  |            |             ^
PTT       _ 3 ____|/|____________  |            |  ____|\|_______ 3 _  PTT
         |        |\| 1N4001    |  |            |  |   |/| 1N4001    |
         |                      |  |            |  |                 |
RX Audio |  4 ___/\/\/\_______  |  |            |  |  __/\/\/\___ 4  | RX Audio
         |       680 Ohm     |  |  |            |  |  | 680 Ohm      |
         |                   |  |  |            |  |  |              |
RF-DCD   |  5 -->To: TNC B   |  |  |            |  |  |To:TNCA <- 5  | RF-DCD
         |           PTT(3)  |  |  |   RADIO    |  |  |   PTT(3)     |
         |------>To: TNC B   |  |  |            |  |  |To:TNCA <-----|
                   RF-DCD(5) |  |  |__TX Audio__|  |  |   RF-DCD(5)
                             |  |                  |  |
                             |  |_____   PTT  _____|  |
                             |                        |
                             |________RX Audio________|
                          
                                       Ground
                                          |
                                          ^

On some radios, you may need to bypass both diodes for proper keying action.
Try it first with the diodes, and if that doesn't work, then jumper them out.
If there are any questions, please send them to the WA8OOH system. 73's

Jeff King    WB8WKA   @ WA8OOH Livonia MI

Via WA8BXN & WD8SCH - 03/20/88
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