Interfacing Tait TB7100 and MMDVM

The other day I got hold of a Tait TB7100 1U repeater. I thought I might make a good basis for a MMDVM HotSpot or even repeater. Apparently the RX had a sort of a problem. Using the service manual I discovered that the TCXO for the RX was missing its power supply due to a removed resistor. After fitting in a replacement the unit worked ok with a testing configuration for a FM repeater.

So that would make a good hardware for a MMDVM HotSpot now. Some research on the Yahoo group revealed that you need to directly wire RX and TX audio directly to the two modules on the inside.

As my version did not have the built-in AC power supply there was enough room to mount the Raspberry Pi and the MMDVM modem onto the bottom of the 19” case. For power supply of the Raspberry Pi I used a 12V-5V DC/DC converter.

As it came down to wiring the MMDVM to the RX/TX modules I took another look into the service manual to identify the possible connectors. RX and TX module both have 15-pin D-Sub connectors (Auxiliary Connector SK101) that are wired to the SI board. The service manual lists the following:

So I made the following connections (Pin numbers on the 15-pin D-Sub connectors of RX and TX-module):

Receiver

RX Audio --> Pin 13
RSSI     --> Pin  6
GND      --> Pin 15

Transmitter

TX Audio --> Pin  7
PTT      --> Pin  1
GND      --> Pin 15

During programming you need to configure GPIO7 as low active and to trigger the PTT.