Bob Ball WB8WGA:

TNC Builders:

I have gotten some more feedback on the TNC article so want to pass it on to everyone 
to expedite your building efforts. Please pardon me if some of this is redundant with earlier emails:

1. The text on page 24 describes the monitor command. 
   In the software implementation, it is actually mon (ie mon off/all/me). Thanks to K1IRK.

2. If you plan to reassemble the software and use MPLAB 7.01 or later you will see 3 undefined symbols 
   due to MicroChip’s unexpected change in name.

   As pointed out by Leo Coleman, this can easily be fixed by making the following changes 
   in the provided tnc .asm file:

   Change TOIE to TMR0IE
   Change TOIF to TMR0IF
   Change _WRT_ENABLE_OFF to _WRT_PROTECT_OFF

3. I have added a reference I inadvertantly missed in the to K5ATM’s open source interrupt service routine. 
   It is now in the source code at the league and I have also asked that my oversight 
   be included in the letters to the editor in the next issue.

4. Phil, K1IRK and Rod Kreuter have experienced some high freq oscillation in the receive circuitry 
   causing the receive led to oscillate. Phil recommends a more robust design would include a slight 
   amount of negative feedback to insure this doesn’t happen. They suggest the following:

   a. Lift ground from Pin 1 of 16F88 (leave ground on pin 18). 
      Insert a 470 ohm resistor from pin 1 to ground.

   b. Insert a 100k resistor from pin 1 to pin 2 on the 16F88.

      This will provide a small of hysteresis and stabilize against oscillations.

   c. They also recommend putting a .001 ufd cap from pin 17 to ground to remove 
      any potential 20 Mhz clock to the comparitor.

      I haven’t experienced this oscillation on any of the 5 units I have built that might 
      have been dumb luck! I think that Phil and Rod have a good idea and plan on modifying 
      one of my units and trying it. Thank you Phil and Rod

5. Phil also has the following suggestion on the transmit circuitry...

   ....I have looked at the output on a scope using the calibration function and it looks ok. 
  I thought it might be a good idea to put a cap across the pot to smooth out the steps on the sine wave 
  approximation rather than letting the radio do that.

  Lastly, wanted to include a photo K1IRK send of his constructed unit. 
  His layout is far superior to mine and shows how the unit footprint can be squeezed down.

6. Rod Kreuter, WA3ENK sent some email about a terminal setup problem he experienced with the TNC . 
   The TNC expects a Line Feed character at the end of a command sequence from the terminal . 
   If it just sees a carriage return for the line end, it echos the individual characters 
   but then times out after about 7 seconds since it thinks the operator went to get a drink in the 
   middle of the command! I use hyperterminal under XP which has the option set to send line feeds 
   (file->properties->settings->ascii setup->“send line end with LF”. 
   Rod experienced the problem with Procomm under DOS. 
   If your experiencing this problem and can’t set your particular terminal program to work please let me know.

   Thanks to Rod for digging into the source code and figuring this one out.
   Nice piece of detective work.

de WB8WGA