Master Tracks Pro has the usual option for sending MIDI data to the modem port. At first I assumed this was for use with C-Lab Export, but actually there's an option which is not available on the Export. It allows you to receive sync through the modem port. The Export has no input of any kind.
If you select "Generate SMPTE", the dialogue title is "SetUp Transport(tm) Interface." So I'm assuming that they made available their own interface for the modem port called Transport? If anyone has any information about this, I would be interested to learn about it.
I managed to find a picture of a different interface called Transport, but it's designed for the Mac. I can't see any way this would work with the Atari, so there must have been a different Atari version: http://www.oldschooldaw.com/forums/inde ... pic=2611.0
What's particularly interesting is how they managed to get input via the modem port. Normally when you're outputting MIDI data, it's nearly impossible to get simultaneous input through the modem port. I'm thinking the Transport interface may have had an SMPTE input, and directly routed the signal to one of the handshaking lines. This would use a certain amount of processor time handling each bit as they arrive, but it would work and the hardware would be very simple.