BobbHH,
While I know nothing of your particular plotter, I'm a Tech, and have been one for almost 20 years.

In all cases I have ever seen, a CHECKSUM error is an indication of a PROM/ROM/EEPROM error. CHECKSUMs are derived by mathematically manipulating the data stored in memory, then comparing it to a last known good value to determine if the data has been corrupted.

Some of the other posts which talk about communication are surely more indicative of a parity error than a checksum error, especially with the symptoms you describe where the error is reported apparently before anything gets passed to the plotter anyway.

I would suggest that you research a method to perform a HARD RESET on this machine, possibly resetting the data stored in non-volatile memory to factory settings. Look for it in a manual, examine the plotter for a set of pins marked RST, or a battery that would have been used for long term storage. Remove said battery for a time and replace, probably with a new one.

Let me know if this helps.

Joe Myers