In your post you mentioned that you just acquired this machine, so have
you run any jobs on it?
Did you see the machine operating before you received it?
I ask because there are a few posiibilities that produce a BUS error....
Is there any other information in the alarm message, such as a code
that may say Y01 , Z01 etc... Are you able to page over to the diag -> alarm
screen and verify if there's any other codes..
Mits alarm page will say something like Z31 DATA ERROR 0001
If you cant page over to the alarm screen, and only see BUS ERROR, that refers
to non-existent memory being accessed... which might mean that system software/ladder has been lost.
If there's a Y code associated with the message, then that means that the MCP has lost connectivity with the drives, check cabling to resolve this.
Another possibility is that the CPU itself has malfunctioned.