Perhaps with luck only a 1 amp fuse that supplies +5V to internal and external terminators is blown, causing the internal terminator to drag down the signals towards ground. See if the SCSI DB-25 pin 25 has +5V on it. On the parallel port devices this is grounded, so the fuse if there is one, or the trace, is blown. Apple uses some unconventional small fuses as well as sometimes some circuit breaker polyswitch types so continuity check any component with a "F" designation on it starting with those in the vicinity of the SCSI connector.