Alpha Microsystems Eagle 100 Specifications download pdf (Page 68)

Page A-3SCSI Termination

A.1.1Termination Procedure (Without External Terminator)

For all Alpha Micro computers without an external terminator, the last SCSI device

attached to the connector farthest down the cable away from the CPU board must have

its terminators installed. If only one SCSI device is installed, that device must be

terminated.

A.2TERMINATION POWER

In order to properly control SCSI bus termination, a termination power source must be

provided; this is especially important when using an external terminator.

Why is Termination Power so important when using an external terminator?

In order for terminators to do their job, they must have a power source. In most (but not

all) cases, a SCSI peripheral will supply termination power to its own on-board

terminators, even if the SCSI host controller or no other SCSI peripheral is supplying

termination power to the SCSI bus. However, for an external terminator to be effective

there has to be termination power supplied to the SCSI bus. If no termination power is

available, the external terminator is not going to do its job, which means your SCSI bus

is not terminated. This may result in a computer that either won’t boot or once booted

may tend to hang frequently. Because SCSI-2 devices transfer data at a higher rate,

they are more prone to display problems when improperly terminated.

The termination power source can be configured in one of two ways, depending on your

application:

AMOS based computers with SCSI-2 implementation (i.e., AM-4000 computers,

AM-540 enhanced AM-3000M computers, Eagle’s and Roadrunner enhanced

computers) should be configured to supply termination power via the host

controller. When termination power is supplied by the host controller, it is not

necessary to have one of your internally mounted SCSI peripherals configured to

supply termination power to the SCSI bus.

AMOS computers using the 50-pin SASI bus, which includes non-SCSI-2

enhanced AM-1000, AM-1200, AM-1400, AM-1600, AM-2000, AM-2000M,

AM-3000, and AM-3000M computers, require that one of the internally mounted

peripherals be configured to supply termination power to the SCSI bus.

SCSI-2 Bus Termination Power Guidelines

Use these guidelines for supplying termination power to AM-4000 computers,

Roadrunner enhanced computers, AM-540 enhanced AM-3000M computers, etc:

1.AMOS based computers with SCSI-2 implementation will be configured with the

host controller supplying termination power to the SCSI bus.

2.When the SCSI host controller is supplying termination power, you want your

internally mounted SCSI peripherals to be configured so they do not supply

termination power.

PDI-00172-60, Rev. A02

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Alpha Microsystems Eagle 100 Specifications Page 68