Modifying the Paccomm TNC-NB96 for 38.4kbps

Created 28 January 2001

Updated 30 January 2001 - converted .tif's to .gif's and added some explanations why certain mods were done the way they were.

Updated 31 January 2001 - RESETing if TNC is performing irrationally

In the documentation for most 9.6kbps G3RUH Modems I've seen has been documentation on upgrading the units for use on other baud rates such as 19.2, 38.4 etc etc. There's now a real reason to do it - satellite downlinks are starting to require 38.4kbps downlinks. Unfortunately, many of the TNC2 clones don't have quite enough oomph to run at 38.4kbps. The Paccomm TNC-NB96 appears to have enough, with a 9.8MHz Z80 CPU. This is the same speed used in the Paccomm Spirit 2. The pertinent differences between a TNC-NB96 and the Spirit 2:

  TNC-NB96 Spirit 2 Satellite 9.6 up 38.4 down
CPU speed 9.8MHz 9.8MHz
CPU Z80 Z80
ROM 64k 64k
RAM 32k 128k
Downlink speed 9,600bps 38,400bps
Uplink speed 9,600bps 9,600bps
Terminal baud rate 38,400bps 57,600bps

So all we're missing is some RAM, higher terminal baud rate and a higher downlink speed.

Well, I don't intend to address the RAM issue here (no pun intended) as this is bank switched in the Spirit 2, and would probably need a firmware upgrade. But I will address the terminal baud rate issue and the downlink speed. For loopback testing purposes I'll also be looking at the implementation of a 38,400bps uplink speed too. This will allow you to check that the system's working without having to wait for a satellite.

To use the RX mode you _will_ need a receiver with an IF bandwidth of 80kHz, and AF bandwidth of 25kHz. Similarly for TX at this rate your transmitter will need setting up in a similar manner. Out of the box almost all radio's will not do this, although you may get some success on the RX portion of the WFM mode of some transceivers. do an IFD board for RX, but I found it rather difficult to install in my FT-847 - your mileage may vary. On the TX side, you may need to do some adjustments and/or surgery to your radio. **Paragraph added 30 January 2001

I recommend performing the mods in the following order: (a) 57,600 terminal rate upgrade, (b) RX 38,400 rate upgrade, and (c) TX 38,400 rate upgrade.

Before starting on the mods, if you can't open up the two PCBs fully flat onto your work area, extend the 20 way ribbon cable so that you can. Without this it'll be nearly impossible to check what's going on.

If you don't have an oscilloscope, I recommend borrowing one for setting this up. A 20MHz unit is more that sufficient for this purpose (that's all I had!).

Before proceeding, please ensure that you can use your unit perfectly on 1200 & especially 9600bps already!!!

You might also like to run the DISPLAY command and print it out so that if the non-volatile RAM settings get upset during the mod and you have to run a RESET command, you know how it was all set up. **Paragraph added 31 January 2001

Standard disclaimer - if you bust your TNC or anything else (including your pride) doing this mod, you're on your own.

Known bug: the DCD light seems to continually flicker in 38,400bps mode.

Parts required

1x 74HCT393 

1x 1.8432MHz Xtal oscillator module (I butchered an old 1200 baud PC modem card for this)

2x 2 way, 6 pole miniature latching push switches - I had to buy these $3 each

1x 33p cap

1x 75pF cap (or 2x 150pF in series)

1x 100pF cap (to replace the one you'll break on the header when you try to disassemble it)

1x 150pF caps

1x 220pF cap (header replacement in case you break the existing one)

2x 330pF caps

1x 470pF cap (header replacement in case you break the existing one)

4x 1nf caps (2 for header replacement in case you break the existing ones)

1x 3.3nF cap (header replacement in case you break the existing one)

1x 10k res

1x 33k res

small piece of perf board

miniature hookup wire

ribbon cable (I butchered an old PC IDE cable)

Upgrading terminal rate to 57,600bps

Internally there's no way of extracting the 921.6kHz required for a 57.6kbps without some extra hardware. In this particular case I used a 1.8432MHz crystal oscillator module which I divided by two with half of a 74HCT393 (see fig 1). The oscillator module came from an old PC 1200 baud modem card which has been sitting doing nothing for the past eight years or so. The 74HCT393 is a generally available part and can be replaced with HC or LS varieties if that's what you have.

Fig. 1: The 57,600bps terminal data rate mod

A track will need cutting at pin 14 of U7 on the TNC board. See Fig. 2.

Fig. 2: Cutting the track on the TNC board at Pin 14 of U7 (74HC4040) on TNC board

I constructed the data rate mod on perfboard (see fig.3). It replaces the action of the 300bps terminal baud rate switch to act as 57,600bps.

Fig. 3: The 57,600bps terminal baud rate mod. The perfboard is upside down and so the chips are hidden. The board is situated on the TNC board. Note the position of JPR/15 as the top left connection in the picture of JPR.

Try the unit out after this first mod, and check that you can operate the unit at the new speed. Things should function identically, except that you no longer have a 300bps terminal rate option: it is now 57,600bps.

If the unit is performing irrationally, especially the TNC not behaving as expected, it's probably worthwhile doing a RESET command to set everything back to factory defaults, but you'll lose any settings you had. I had to do this on one occasion when things just weren't quite right - all CAPS for instance amongst other spurious things. **Paragraph added 31 January 2001

RX upgrade to 38,400kbps

The schematic for this is in Fig. 1. You'll need to break tracks under JP20/1&2 and JP21/1&2 on the G3RUH board - see Fig. 4. These two jumpers control the RX data rate. **Paragraph changed 30 January 2001

Fig. 4a: The 38,400bps RX mod: switches are shown in 9,600bps mode


Fig. 4: Cutting the tracks on JP20 & 21 of the G3RUH modem board

Remove the jumpers from JP20 and JP21 on the G3RUH board also.

Assemble the switches onto the G3RUH board as shown in Fig.5 using a glue gun to affix correctly. Drill holes to accommodate the switches (see Fig 6.). The position of the switches is a tough one - it's certainly not easy to find space once the boards are sandwiched together.

Fig.5: Position of switches for switching in the 38.4kbps modes to RX and TX independently


Fig. 6: Where the switches appear on the front panel

Wire up the RX switch to the header on the G3RUH board, with the new components mounted on the header itself. I found that when removing some of the header components they broke, so be prepared with some replacements. You may find it easier to simply snip one end of the capacitors rather than desolder them. The resistor remains in place on the header, with the 33k being switched in in parallel.

Wire up the RX switch to J20 & J21 on the G3RUH board. Add the additional modifications to the other half of 74HCT393 on the perfboard on the TNC board. **Paragraph changed 30 January 2001

TX upgrade to 38,400kbps

The schematic for this is in Fig. 7.

Fig. 7: TX mod to 38,400bps - switches are shown in 9600bps mode

You'll need to break a track on U9 between pins 1 & 2 (fig 8), and put in a 10k pull up resistor between pins 1 and 14. This particular part of the modification fools the 9600 TX modem detector filter into thinking all's still hunky dory at 38,400bps. Without it, all will fail at 38,400! **Paragraph changed 30 January 2001

Fig. 8: Cutting track between pins 1 & 2 of U9 on G3RUH board, and position of 10k pull up resistor

As well as wiring the three header capacitor switch replacements in the schematic of Fig 7, C32 (near the header) needs wiring to the switch too. Remove the JPCLK jumper, and wire up that part of the switch shown in the schematic. Wire up pin 1 of U9 to the switch to pull to ground when in 38,400bps mode.

Fig. 9: The TNC board after mods


Fig. 10: The G3RUH board after mods (yuch)


To test, after doing the mod check that your TNC-NB96 works as before in all its previous modes - 1200 AFSK, 9600 FSK and if you have any external modems like a PSK-1 for example, check that they still work OK.

Remember that you may need to issue a RESET command to set everything back to factory defaults if the TNC doesn't seem to behave quite right. **Paragraph added 31 January 2001

Install link JP6 on the G3RUH modem (loopback). Switch to DFM mode. Switch on both 38k4 switches (RX + TX). The DCD lights should be on permanently.

Run the command FULLDUP ON

Set up your callsign if it's not already set up, eg MYCALL G6LVB

Connect to yourself: C G6LVB

Check you can chatter away to yourself: everything you type should be repeated when you hit enter.

Connect to your radio (which of course you have set up for wide band 80kHz FM RX) and away you go.

A more expensive way to test the Paccomm Mod

Mail Howard, G6LVB