Modifying the PacComm Tiny-2 TNC for
Use at 19.2kB with the D4-10 Radio

We've used a number of PacComm Tiny-2 TNCs (TNC-2 clones) with the Kantronics D4-10 radios at 19.2kB in "RF Modem" mode (i.e., not using a G3RUH-style modem).

The hookup is pretty straightforward with one gotcha. Here are the steps we used:

1. Convert the TNC to 10MHz CPU operation. The easiest way to do this is to either buy the TNC from PacComm with the 10MHz option ($25 extra) or buy the parts kit from PacComm (which also costs $25, and which can be used to upgrade other TNC-2 clones as well) and follow the instructions given there.

2. You will need to run five wires between the TTL I/O connector (DB-9) on the D4-10, and the guts of the Tiny-2. It's probably easiest to wire the Tiny-2 so that you can use the 5 pin DIN connector on the TNC -- three of the signals you need are already there.

a. Find the 20 pin modem disconnect header inside the Tiny-2. Make sure that pin pairs 17-18 and 19-20 are not shorted together; use a knife to cut the traces if necessary.

b. Route TTL TXData to pin 1 of the DIN connector. Run a wire from modem disconnect pin 19 to pin 1 of the DIN connector. An easy way to do this is to remove C8 and run the jumper to the side of that cap that went to pin 1. Removing C8 isolates the TXData line from the other audio circuits in the TNC.

c. Route TTL RXData from pin 4 of the DIN connector to modem disconnect header pin 17. You can lift R28 (? -- the number isn't clear on the schematic, but it's 100 ohms with one side going directly to pin 4) to isolate and inject the signal.

d. Use DIN connector pins 3 (PTT) and 2 (ground) as normal. Connect the DCD signal from the D4-10 to DIN pin 5.

e. The D4-10 DCD signal is opposite the sense the TNC needs, so some magic has to be done. The easiest way is to: remove U16 (the 3105 modem chip -- save it because they're getting difficult to find!), short pins 2 and 3 of U17 together, and make sure that jumper "JPD" is shorted across pins 1 and 2 (not 2 and 3 as you might suspect).

f. Set the radio port speed to 19.2kb, and the serial port speed to 38.4kb, and you're done with the hardware mod.

g. We like to use the "KISS56" eprom image that is available from the GRAPES group. It's optimized for higher speed, and it has the real advantage of using 2.5ms timer ticks instead of 10ms ones, so you can set the TXD and slottime values much more precisely. As Jerry Pournelle would say, "highly recommended."

Note: Step "e" is necessary because simply feeding the DCD signal from the D4-10 into pin 1 (the DCD input) of the modem disconnect header doesn't work. The DCD LED on the TNC will properly follow the D4 squelch, but it won't actually tell the TNC there's a signal present -- the Tiny-2 will act as if it's in full duplex mode. This is because the DCD LED loads down the signal to the point that the logic level isn't correct. The modification routes the signal through a couple of buffers to give it enough oomph to work properly.

Return to Ham Documents