Thanks to both of you for the advice. I finally gave up on the ST15150 as a chain end device and put an ST34371WC with a 50 pin adapter there, and the ST 15150 is in the middle of the chain. All is well with that configuration.

The only 50 pin terminator I have is one of the Centronics 50 external types but will have a look at e-Bay for some that will terminate a ribbon cable, for future use.

This SCSI stuff is kind of like networking; there's a little bit of magic involved and if you don't hold your mouth just right things don't work.