[Slothie]

I've currently got some "debug mode" code in there that allows me to switch the various signals on and off, and read or program bits - so far I've just been monitoring the signals with my bitscope and logic analyser to check them, I haven't risked a chip on it yet given the number of faults I've been uncovering.

I was originally going to just connect to a terminal program as all the I/O is currently occupied and there's be nothing for LCD displays or SD cards. One of the problems is that when trying to upload an intel hex file to memory characters are being lost, although one answer would be to slow the baud rate to below 9600, its not like we're going to need a high speed.

The other idea is to write a python or C application to connect to the low level API across the serial link and drive the programmer without having to upload anything.

A custom sketch that just calls a library to do the programming functions would also be another option as you suggest. This would require the user to have a higher level of knowledge, although I doubt a rank amateur would be fiddling with bipolar PROMS.

Of course, on a redesigned device with a different ATMEGA or an I/O expander there would be plenty of I/O for LCD displays, SD cards and buttons on a standalone unit but we are hitting feature creep on a device for programming an obsolete device...