A generous customer gave me an old Compaq 7 disk array and a single port UltraSCSI module to troubleshoot some RAID5 configurations, here are the switch options to set provided by Alex Daniels.
Module: DS-BA35X-FA is a Single port, single-ended UltraSCSI personality module
for BA356. Although these settings cover the single or dual
port(DS-BA35X-FB) variants.
It has two sets of switches as you have observed (S3 and S4).
S3 (SCSI Address switch) – When shelf bus address switches on shelf one are all set to OFF, then the SCSI bus IDs are 0 through 6 on the first shelf.
When shelf bus address switches 1, 2, and 3 on shelf 2 are set to ON, and 4,
5, 6, and 7 are set to OFF, then SCSI bus IDs are 8 through 14.
S4 (Bus termination switch) – 1,2 are for Channel A. 3,4 are for Channel B
1 OFF or 3 OFF – 16 Bit External Bus Termination
Terminates all SCSI bus bits (full termination). This configuration is used when the shelf is the end point in a SCSI bus and terminates all signals.
1 ON, 2 OFF or 3 ON, 4 OFF – 8 Bit External Bus Termination
Terminates only the upper 8 data. This configuration is used when the input bus is a wide (16-bit) bus and it continues as a narrow (8-bit) bus. This could be used with a tri-link to a narrow device.
1,2 ON or 3,4 ON – External Bus Expansion
No termination on the shelf. The shelf is not an end point of a SCSI bus segment and no termination is needed.