[WaveyDipole]

Fair enough. It is possible to set the carrier frequency and level and the display is working. There is a response to all key presses so I agree, the CPU must be running.

Through trial and error I have discovered that GPIB does actually work! For some reason the #55 sequence which is supposed to display the GPIB address always shows 16 instead of the current GPIB address. The actual address was set to something else. I have confirmed that the instrument can communicate on the address that is set with NN#55 sequence where NN is the desired address. The address is stored between power cycles but if not set to 16, one has to remember what it is set it to! I have now set it to 16.

Since the manual states that the terminator is a carriage return, it is necessary to set the terminator as carriage return only on the GPIB adapter. The instrument then accepts commands as described in the manual. Also as described, commands are executed just as though the corresponding key had been pressed.

Unfortunately %AM and kHzFM still do not work. They behave exactly the same as pressing the corresponding button and just return 'ignored'. This would seem to rule out problems with the keyboard membrane and the decoding of key-presses by the processor and would tend to suggest that the problem is with how the processor executes the command, or at least with how that command translated from the processor to the RF generator unit. There are a number of 74xx series and the odd CMOS ICs that form the switching logic between the CPU and the RF generator. That is where my next efforts will be focused.

Still a way to go, but a little progress made.