Retro: PIC Based TNC

Here are my notes for a PIC-based TNC, built (mostly) to the design of Bob Ball WB8WGA.

The original project documentation may be lost, but it described an inexpensive ($25) TNC using a PIC microcontroller. If you are going to build one then as well as the components you will need a PIC programmer and the firmware. I have modified WB8WGA’s design slightly based in part on the
modifications of a German ham, Klaus H. Hirschelmann (http://www.kh-gps.de/minitnc.htm). He removed the MAX232 level shifter and replaced it with a pair of buffer transistors, and added hysteresis on the comparator circuit. This change (and other useful points) is explained in the errata/update file here (http://forum.qrz.ru/attachment.php?attachmentid=6945&d=1176928078).

To make construction easier I have drawn a stripboard layout (below).

The modifications to WB8WGA’s original design are:

MAX232 replaced with buffer transistors
There is a 1nF cap on the audio input
There is a feedback circuit on the comparator
There is a jumper to allow PTT to be disconnected from Audio Out There is a jumper to allow +5V direct supply (bypasses the regulator and protection diode)
There is a jumper to optionally supply +5V output to power a GPS

I have also included a proposed pinout for RJ11 connectors to be used for the serial interface and the radio interface. I have noticed that many of the APRS devices must be connected with a DB9 plug. The backshell for the plug is huge relative to the device, and often the plug does not have retaining screws fitted. Since only four signals are required for either connector (serial or radio) I chose RJ11 connectors for their small size and latching ability.

Construction notes

The layout is shown on a 0.1″ stripboard grid. Tracks are running horizontally across the board. Please read the original documentation and collect your components together. I recommend using an IC socket for the PIC, and to test- fit all components before soldering. On the diagram a line indicates a wire jumper, and an X indicates a track break. Use a track-breaking tool or small drill bit to break the track at the hole. There are two track breaks between holes, shown as || near the crystal/resonator. Use a craft knife to carefully cut a thin piece out of the track between the holes. The particular components should be obvious, but if I get too many reports of construction difficulty I will improve the documentation.

I tried to fit the components into the smallest area possible. Feel free to grow the board to fit whatever box you plan to use, and to alter the mounting holes as required. Also, please note that the wire links on the right hand side of the board are mostly to re-order the I/O points. By all means rearrange these to suit your connectors. I have drawn the regulator as a full- size 7805. These have a larger thermal mass than the small 78L05 devices, so even though power dissipation is low due to the low current consumption of the circuit I prefer the full-size package.

Finally, I take absolutely no responsibility if you build this circuit with my layout. I have inspected the layout, but at the time of writing (January 2008) I have not actually built it, so I cannot say that it will work. It is up to you to understand the circuit, confirm the layout is correct, build it properly and test it before hooking it up to your expensive and delicate ham rig. I take no responsibility for errors in my layout or your construction. Sorry.

Update 2008-10-30: I have started building one. The board is 91x56mm because of the connectors I chose to use. I haven’t programmed the PIC yet.

Update 2008-11-6: Finished and working!

2015/01/30 | Categories: amateur radio | Tags: amateur radio, aprs, PIC, WB8WGA | Leave a comment