EDM (standard Detroit Electonic Display Module) as designed for he DDEC system is wired into the 900/901 can bus (which is J1587 data link) it can 'drop out' when DDDL (diagnostic tooling) is connected into the same link etc...our recommendations are to kill the EDM power (simply unplug connection at rear of EDM) if plugging in DDDL. 

      Most installations we've done of EDM (ie. excavtors etc.) i've actually recommended the EDM actually be wired to the Diagnostic connector plug (as all needed is there) and it then forces technician to remove the EDM connector to plug in the DDDL etc... ensures there's no corruption whilst DDDL trying to communicate.