I finally determined after I got the boxes all working that since they weren’t going to be returned I’d go ahead and solder in a 10pin header for the serial port. This requires removing the circuit board from the box. I put in all 10 pins even though that makes it almost impossible to actually connect a header to the pins as the front panel LEDs are right against them. You should probably put down 6 pins instead (5-10) as it’ll be easier to work with. I changed my prototype external circuit to use the 3.3v in the header instead of an external supply, which worked very well. In the final circuit, I used an ECG211 transistor (Emitter to pin 9; base via 270ohm or higher to pin 10) as the output driver to the PC serial (PC data in to collector) – I didn’t need a negative supply on one laptop, but did with a 100ohm to data out of a 2nd serial port for other PCs, I suspect that 2 1.5v cells or a 3v watch battery would have provided this very well. (The data out of the same serial port is usable, but expect garbage to be received by the PC when typing.)
For the data out from PC to the Ericsson box, I used an ECG210 transistor (Emitter to ground; base via 270ohn or higher to the PCs data out) with collector directly to pin 8 on the Ericsson header. There is no negative voltage here as the inputs and outputs of the Ericsson box are 3.3v TTL. I did not get a chance to check out the internal wiring or voltages of the G4 box (with a connector on the back) as I shipped it out before getting to it.
Final parts rundown, 1 PNP, 1 NPN, 2 270ohm or higher resistors, plus an additional 100ohm for most usages. Use a PC with 2 serial ports and you’re in business. For production grade usage, you should have a reverse protection diode on the PC to Ericsson transistor’s base – I skipped that here 🙂