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Rs232/USB 1200 Baud Modem

FoxDigi: 1200 Baud Mini-TNC/Modem: Project+Kits

VixenTrak: GPS-less Position Encoder for FoxView2

Hamcom Interface : Project & Kits

APRS Trackers & Viewers




 FoxDigi: A 1200 Baud Radio Modem using PIC16F88


A Stand Alone, most economical,  APRS Digipeater.

This project is revised and listed here.......

 Yahoooo!!! Project is complete !! 


Some Examples of FoxDigi in operation:





Click to see live data  

Click to enlarge


 FoxDigi  Construction by Radio Amateurs
E20EHQ / KB3KCI Home Brew Web Page



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Although Sound Card Interfaces are already there with good amount of free Sound Card software, there are always reasons to use a dedicated modem.

Earlier project using MX614 was very successful and many radio amateurs built RM1 USB/RS232 Modem Kits.

MX614 is a low voltage, low current modem chip. Excellent chip for a simple low current 1200baud Modem/TNC. However, keeping in view the high cost of this chip ($9+Postage!!) and  recent unavailability with most suppliers, gives us a good cause to think of alternatives. Result was this PIC based modem/TNC to get similar results. In fact, this project achieved more than MX614 by way of having capability to store user data and APRS frames.

FoxDigi has following point of interest:

1. A Simple 1200 Baud TNC

2. A stand-Alone APRS Digipeater.

3. An APRS Tracker with GPS Support

4. Works on APRS, with or without a GPS.


However, In this project, I paid special attention to get an economical APRS Digi, that too without using a GPS. To achieve this, only a command statement of Location (Lat/Long.) is required to be included in BTEXT.

FoxDigi has two D9 connectors for outside communications. J1, a D9M is provided for your transceiver. J2, a D9F is for connection to your PC.

FoxDigi is required to suppy +12V (+7-12V) from Transceiver connector J1. An internal regulator (78L05) provides +5V for digital circuits. Current consumption is well below 20ma (Depending on LEDs) which is good enough for most remote solar powered locations.

FoxDigi uses a standard RS232 driver to interface with your PC (or GPS) J2 is configured as DCE using D9F connector. A null modem will be required for GPS. However, it should be noted that this project is not designed for use like a tracker but rather a Stand alone APRS Digipeater.

Project is designed on 8cm X 6cm Double Sided PTH board and results were very encouraging. With this modem, perhaps, you will never have to worry about MX614 for your projects. 

At moment, two firmware are available: 9600 & 4800 baud. This is only important if you wish to use a GPS with FoxDigi. Firmware Baud Rate will effect both, the GPS & the "Hyperterminal" used for config of your FoxDigi.

In a stand-alone Digipeater application, a BTEXT beacon with your position should be good enough. As the current consumption of a GPS is perhaps too large to handle at a remote site.

Picture of the completed 1200 Baud Modem:

Click the image to enlarge


FoxDigi Document

1200 baud modem using MX614

Schematic & Parts List

1200 baud modem using MX614

Configuration Document

1200 baud modem using MX614

Using Hyper Terminal of Windows

1200 baud modem using MX614






Want to make your own PCB? Gerber files are here....

Single Sided PCB Files.

Component Datasheets

Datasheets for this project parts



With this modem, you can use many software available either as a freeware or shareware. Please go thru following links & find one suitable to your communication requirements.
Digital Communication Software Collection
Digital Communication Software links
Understanding Digital Communication




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