Modifying the MFJ-1270C TNC for
Use at 19.2kB with the D4-10 Radio
Although the PacComm Tiny-2 has been the primary TNC we've used for 19.2kbps operation with the Kantronics D4-10 radios, we've also put a couple of MFJ 1270C TNCs on line as well. The circuitry of the 1270C is somewhat different than the very common "B" models, and although I've modified a couple, I don't have a schematic to verify whether what I've done is optimal. Nonetheless, it works...
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 buy the parts kit from PacComm that's intended to upgrade the Tiny-2 TNC to a "node" model (cost approximately $25), and follow the instructions provided by PacComm.
2. To operate the radio port at 19.2kb, and the serial port at 38.4kb (to minimize latency, it's best to run the serial port at least twice as fast as the radio port). To get those speeds from the TNC:
a. bend out pins 2 and 12 of U1.
b. jumper U1 pin 2 to U1 pin 3, and jumper U1 pin 12 to U1 pin 14.
c. unsolder dipswitch pin 7 and jumper it to dipswitch pin 4.
Setting switch 1 "on" provides 38.4kb on the serial port, and turning switch 8 on provides 19.2kb on the radio port.
3. Replace serial port driver U3 (an LM748) with either a TL084 or LF347. This is critical because the LM748 isn't fast enough to operate faster than 9600 baud (and even there it sometimes stumbles).
4. You will need to run five wires between the TTL I/O connector (DB-9) on the D4-10, and the guts of the 1270C. It's probably easiest to wire things 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. You can pick up the signal at the outer pin (nearest the circuit board edge) of JMP-L or JMP-R and route it to pin 20 (NOTE: this is supposed to be pin 19, but it appears that the connections are reversed on the board!!!) of the modem disconnect header. You will need to lift one side of both C94 and C97 to eliminate their filtering effect; otherwise, the square waves generated by the TNC will look pretty crummy by the time they get to the DIN connector.
c. TTL RXData comes in on pin 4 of the DIN connector. You can pick the signal up on JMP-R, using the pin furthest from the side of the circuit board, and then route it to modem disconnect header pin 17 with a jumper wire. As with the TXData line, you need to bypass the analog filtering components to preserve the quality of the TTL signal. To do this, lift the end of R113 that's closest to the front of the board.
d. Use DIN connector pins 3 (PTT) and 2 (ground) as normal. Connect the DCD signal from the D4-10 to DIN pin 5.
5. The D4-10 DCD signal is annoying -- because it's generated by an op-amp rather than a real TTL gate, it doesn't pull close enough to ground to reliably interface with other devices. There's an easy fix for the 1270C, though. J3 is intended to activate or deactivate the external DCD line on pin 5 of the DIN connctor. It provides a convenient place to install a transistor switch that buffers the DCD signal and allows it to work properly. Here's what you do:
a. Find a PNP transistor (a 2N2907 or 2N3906 will work fine);
b. Solder the collector (the right-most pin with the flat side up and the leads pointing toward you) to the pin of J3 that's closest to the side of the circuit board.
c. Solder the base (the center pin) to the other pin of J3.
d. Solder the emitter (the left pin) to ground. There's a small solder pad near the end of R93 that will work; I used a piece of wirewrap wire to get from that pad to the transistor lead.
6. 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."