LSI SYM22801 - Manual (Page 33)

SCSI Bus Termination 2-19
The peripheral device terminators are usually set with jumpers, resistor
modules, or with a switch on the peripheral. Refer to the peripheral
manufacturer’s instructions and to the user’s manual
for your computer
for information on how to identify the terminator setting of each device
and how to change it.
Caution: The auto-enable/disable sensing feature on the SYM22801
may enable termination erroneously if it is directly cabled to
another SCSI device or host adapter using the same
sensing method. This is dealt with by properly setting the
termination override switches U1 or U2 (see Figure 2.1) as
explained in this section under making an internal
connection with another SYM22801.
The SYM22801 senses SCSI devices by detecting the ground signal on
pin-22 of a 50-conductor internal SCSI ribbon cable, or pin-50 of an
internal or external 68-conductor SCSI cable. If you use an adapter to
connect 8-bit devices externally with a 50-conductor SCSI cable, the
ground signal is detected on pin-34.
The SYM22801 automatically covers SCSI bus termination for five
different bus configurations, depending on the use of the connectors on
the SYM22801 host adapter (see Figure 2.1). The five bus configurations
are:
•
Only for internal SCSI connections,
•
Only for external SCSI connections,
•
For both internal and external SCSI connections,
•
For connections to both internal connectors, and
•
For an internal connection to another SYM22801, or any connection
to a device using the same sensing method for automatic termination
as your SYM22801.
2.3.1 Internal SCSI Connections
If only internal SCSI device connections to the host adapter have been
made, you must terminate the last internal device on the SCSI bus. You
must disable the termination on all other devices. Termination on the host
adapter is automatically enabled in this case.