Adaptec AHA-2742T User Manual - Page 36

Termination on the SCSI Bus

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Getting Started Termination on the SCSI Bus To reduce signal reflections on the SCSI bus, the first and last physical SCSI devices on the ends of the SCSI bus must have a set of resistors called terminators either installed or enabled. All other SCSI devices installed between the ends of the SCSI bus must have their terminators either removed or disabled. Improper termination of the SCSI bus results in erratic device operation or general bus failure. Table 2-1 describes the three possible SCSI device and Adaptec Host Adapter (AHA) configurations. Table 2-1. SCSI Device and AHA Configuration Devices Connected To Host Adapter Internal and External devices (AHA in between) Internal devices only (AHA at end of cable) External devices only (AHA at end of cable) AHA Termination Removed/Disabled Installed/Enabled Installed/Enabled Internal and External Devices When both internal and external SCSI devices are connected to the host adapter, the last external device and the last internal device should be terminated, as shown in Figure 2-2. INTERNAL DEVICE NO TERMINATION INTERNAL DEVICE TERMINATION ENABLED HOST ADAPTER NO TERMINATION EXTERNAL DEVICE NO TERMINATION EXTERNAL DEVICE NO TERMINATION EXTERNAL DEVICE TERMINATION ENABLED Figure 2-2. Internal and External Devices 2-9 Adaptec AHA-2740 Series User's Manual Stock Number: 510381-00 / Rev. C (Page 2-9) Print Spec Number: 491709-00 Rev C Current Date: 9/1/93 ECN Date: 9/14/93

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Termination on the SCSI Bus
To reduce signal reflections on the SCSI bus, the first and last physi-
cal SCSI devices on the ends of the SCSI bus must have a set of
resistors called
terminators
either installed or enabled. All other
SCSI devices installed between the ends of the SCSI bus must have
their terminators either removed or disabled. Improper termination
of the SCSI bus results in erratic device operation or general bus fail-
ure. Table 2-1 describes the three possible SCSI device and Adaptec
Host Adapter (AHA) configurations.
Table 2-1. SCSI Device and AHA Configuration
Devices Connected To Host Adapter
AHA Termination
Internal and External devices (AHA in between)
Removed/Disabled
Internal devices only (AHA at end of cable)
Installed/Enabled
External devices only (AHA at end of cable)
Installed/Enabled
Internal and External Devices
When both internal and external SCSI devices are connected to
the host adapter, the last external device and the last internal
device should be terminated, as shown in Figure 2-2.
EXTERNAL DEVICE
NO TERMINATION
EXTERNAL DEVICE
NO TERMINATION
EXTERNAL DEVICE
TERMINATION
ENABLED
HOST ADAPTER
NO TERMINATION
INTERNAL DEVICE
NO TERMINATION
INTERNAL DEVICE
TERMINATION
ENABLED
Figure 2-2.
Internal and External Devices
Adaptec AHA-2740 Series User’s Manual
Stock Number:
510381-00
/
Rev. C
(Page 2-9)
Print Spec Number:
491709-00 Rev C
Current Date:
9/1/93
ECN Date:
9/14/93
Getting Started
2-9