HP 400 Series Servicing Manual page 63

The SCSI interface uses a single-ended (versus differential) connection to daisy-
chained devices. The SCSI controller provides the interface between the SCSI bus and
the DIO II bus, and can use 32-bit DMA on the DIO II bus.
The length of the SCSI bus cable must not exceed 6 meters (19.6 feet). This maximum
cable length includes the length of the bus inside the Series 400 system units and the
length of cable inside each external SCSI device. For Model 400t and 400dl systems,
the internal SCSI bus length is 0.6 meters (2 feet). For Model 400s systems, the inter-
nal SCSI bus length is 1.6 meters (5.5 feet). For more information about calculating the
length of the SCSI cable, refer to the appropriate owner's guide:
• HP Apollo 9000 Series 400 Workstation Domain/OS Owner's Guide
• HP Apollo 9000 Series 400 Workstation HP-UX Owner's Guide
For bus termination on Model 400s systems, a terminator must be present at each un-
connected end of the SCSI interface. One terminator is permanently connected to the
end of the internal SCSI interface cable used for internal mass storage devices. If the
external SCSI connector on the CPU board does not have a cable connected, a termi-
nator must be installed on the connector. In all cases, the last external SCSI device
must have a terminator in its unused connector.
For bus termination on Model 400t and 400dl systems, a SCSI bus terminator is always
present on the CPU board, regardless of a system's configuration. If there is no exter-
nal SCSI device, a terminator is not necessary on the system unit's external SCSI con-
nector. If there are external SCSI devices connected to the system's external SCSI con-
nector, the last external SCSI device must have a terminator on its unused connector.
For systems that use HP-UX Compatible mode, you can use the Boot ROM Interface
Configuration Mode to select the following SCSI parameters:
• Select code from 0 to 31
• Interrupt level from 3 to 6
• Parity checking on or off
• Bus address from 0 to 7 (normally set to 7)
For information about using the Interface Configuration Mode, refer to Appendix C.
Centronics Parallel Interlace
This interface provides bidirectional support for external parallel devices. It uses an
8-bit Direct Memory Addressing (DMA) scheme to the DIO bus at up to 300 KB per
second transfer rates.
Theory of Operation 3-13