Running a TNC-2 clone at 19K
                ============================
 
The following is a relatively easy modification that allows reliable 19.2K baud on the serial port and radio port of most TAPR TNC-2 boxes.

***** FIRST AND FOREMOST *****

Replace U3 (LM324 op amp) with a TL084CN or TL094 if no TL084CN or equivalent. This is a high speed JFET hi-Z op amp.  Just that fix alone will allow you to run 9600 baud on the serial port to your computer.  I found that the TAPR 1.1.6. code WITH the PMS ran without any serial port misbehaving BUT it was sluggish.  The 1.1.6. code (or lower) without PMS is quite a bit quicker.

***** 19.2 K *****

Cut the PC trace that goes to pin 2 of divider U1 about 1/8" from U1. Run a short jumper from pin 10 of the divider U1 to the side of the trace you just cut that goes to the SW2 baud rate switch.  This is the side of the cut opposite to pin 2 on U1.  This supplies a 307.2K clock to pin 1 and pin 6 of the SW2 which NO LONGER SELECTS 300 BAUD.

The 307.2 K is 19.2K/16 which is fine and dandy.  To run 19.2K on the serial port, turn SW2 switch 1 on.  SW2 switch 6 selects 19.2 K to the MODEM port, switch 7 selects 1200 baud to the modem port and switch 8 selects 9600 baud to the modem port.

(Keep in mind to leave switch 7 on unless you have installed a higher speed modem in your TNC.)

We are running 19.2K serial and 9600 baud radio with G3RUH modems and
Kantronics DVR 2-2 radios with excellent results.  Next step is to up the G3RUH boards to 19.2K by replacing a couple of capacitors.

Anthony Varga VE3ZAV


               Improving TNC2 clone filtering
               ==============================

In August 1988 issue of Packet Status Register, on page 21, the following information is given:

"If you have a TNC-2 or Clone (except MFJ-1274), and have not already removed the MF-10 filter and associated header parts, do so at this time. The reason for this is that the operation of this filter circuit is marginal at the supply voltages used in the TNC-2 and this drastically reduces the modem's dynamic range.
This is a very easy modification. Simply remove both the MF-10 and the header associated with the MF-10. Then under the board solder a jumper between pins 1 and 8 of the header socket. Removing the MF-10 also unloads the -5 volt supply, improving its regulation and reducing the noise generated by the charge pump circuit.
There is ABSOLUTELY NO PERFORMANCE PENALTY for removing this filter."

73 Odd-Egil  LA8AW @ LA8D


PORTED AND REALIGNED FROM PC TO AMIGA FORMAT BY BOB G0LBQ - N6XDI.