I am not sure just HOW you are trying to make your M100 talk to your CoCo,
but…
I have had NO difficulty getting my M100 to talk to my CoCo over a null
modem cable using TELCOM on the M100 and Mikeyterm on the CoCo at 300 baud.
This has worked perfectly for a number of file transfers. If you need greater
speed or accruacy, Disk Plus, although a pain to use due to PCSG's unfortunate
choice of Kilgus DOS, is a solid and effective 9600 baud error checking error
correcting transfer utility that works between CoCo and M100.