IBM 2104 Model DU3 - Manual (Page 63)

Dual-Bus Mode Configurations
For a dual-bus mode configuration, the SCSI Bus Split switches on the switch card
of a 2104 Model DU3 must be set to On.
Each 2104 Model DU3 in the configuration is connected either to one external SCSI
connector on an adapter card, or to an integrated SCSI port. If an adapter card has
four external SCSI connectors, it can be connected to up to four 2104s (Model
DU3).
The external SCSI connectors can be either on the same adapter card, or on two
separate adapter cards. If the connectors are on two separate adapter cards, these
adapter cards can be either both be in the same using system or be in two
separate using systems.
Appendix A. Operating with RISC Systems 47