First, thank you very much. That manual covers the commands, the manual that I got with the unit which I purchased new in about 2003/4 came with a paper manual that did not cover the commands, and the pdfs I had found on-line matched my limited manual, however this G5100 does have a DB9, and a setup menu on the front panel that sets baud rate.
My Firmware version is 7.01 which shows in the panel display, and shows serial #G000721.
I am using minicom, and I have confirmed that the G5100 is echoing characters at 9600 baud, and if I change the baud rate it does not echo, so it is not echoing in any kind of hardware loop back, the characters are being received and the firmware is echoing them back. I also observe that if I turn on hardware flow control in minicom the function generator does not echo, not sure what that means, probably that they didn't implement it. When hardware flow control is off it does echo. The only thing the front panel allows you to do is to set the baud rate, 9600 is the top rate.
*IDN? does not respond with the Model information that the manual describes. No commands respond that I have tried.
So, it is a mystery. I'll pop it open and see if there is anything to see there. Maybe the firmware version is an early one that had some bug.
Any suggestions or theories are welcome, and thanks again for the manual. It has allowed me to know what "Should Work".
I don't have any schematic or service info, I'll see if there is an EPROM inside, maybe I need a later firmware version.
As a hobby, I fix old radios and this signal generator has a sweep function for checking bandwidth and aligning, and the front panel is a bit awkward to use, serial control would be a big improvement.