Well not a real update as such, but an interesting milestone.
Using the MightyCore package I’ve managed to get the Arduino bootloader up on one of these boards. Using a USB ASP the bootloader flashed ok. There is something screwy with the serial port bits as the baud rate and speeds are way out of whack. I suspect its an oscillator setup issue as the timing loops are running way too fast, Aprox 7 times too fast and given we have a 7 Mhz Oscilator and arduino thinks we are running at 1Mhz its a safe bet. I’ll have a pop at changing the 7MHz osc for an 8Mhz and see what can be done to get this happy.
Still it means that in theory all the pins are available to me. I have the top drivers traced out. I’ll work out how to make the resultant drawing legible at some point.
It also turns out that the serial number is in the ATmega’s EEPROM. This would imply that the modem is a complete stand alone unit that’s simply sending and receiving data, kinda leaves the path open to making these talk to each other.