Only a single RX50 pair is allowed, and no hard disk. The graphics board and console video interface can share a single color or mono monitor (unlike the DECmate II which always requires the mono monitor). 

The printer interface baud rate cannot be changed from the fixed rate of 4800, but is otherwise compatible.

The communications option is compatibly upgraded to include support for an optional 300/1200 baud DEC "Scholar" compatible internal modem. Certain programming constraints must be applied when accessing the comm. chip that makes certain existing OS/278 programs incompatible with the DM III, but this is fixable.

All video and real-time interfaces are compatible.