Data on the Syntrx
One of the challenges of using digital modes such as SSTV, RTTY and Packet, is interfacing your computer to your radio. Often you can just unplug the microphone and plug the transmit interface into the mic socket, but what happens with radio’s like the Motorola Syntrx where the microphone also contains the power and channel change functions?
Fortunately with the Motorola Syntrx there is a simple solution that allows you to use the radio for both voice and data, without the need for complicated wiring solutions or even swapping of interfaces. Inside the Syntrx, with the microphone connector facing you, locate the microprocessor - U402. It is the large IC located just below the PROM/EPROM IC. Below the microprocessor there are 2 rows of 20 solder pads, labelled P100. This is the “Option” interface for the Syntrx.
The layout of these pins is shown in the picture above, with the relevant pins numbered. The pin functions are:
Pin 3 - Rx Data (sound from the radio to the computer)
Pin 23 - Ground
Pin 29 - PTT (ground this to transmit)
Pin 30 - Tx Data (sound from the computer to the radio)
Pin 32 - Ground
So what’s so different about this interface? Firstly it allows you to still use the radio for normal voice operation using the microphone. You don’t lose any of the microphone functions or have to run complicated wiring. Second the Tx Data input aboce is a specially for data rather than voice, so it avoids the pre-emphasis and amplification done to the voice audio input. And third, yep the Rx Data is a special data output that comes straight from the audio pre-amp before de-emphasis and amplification, or even mute circuitry. Also, the PTT function operates independant of the microphone PTT.
So, what type of interface to use for SSTV/RTTY/Packet? If you use a programme that uses your computer sound card, the interface is very, very simple. Programmes like MMSSTV, MMTY and AGW use the soundcard to receive and send the data, and the PC “Com” port to trigger the PTT function. All you need is a few components to interface between the computer and radio. The pins on the diagram below are numbered to match the P100 interface on the Syntrx, as detailed above.
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