ATTO ExpressPCI UL4S User's Guide download pdf (Page 13)

8

Cabling and termination

• Use an LVD terminator if you are only using LVD

devices.

Although you can use a Single-Ended

terminator, all devices will be limited to Ultra

SCSI speeds. Single-Ended devices require

a Single-Ended terminator.

If you use an LVD terminator with Single-

Ended devices, the system may hang or

devices may not be seen on the SCSI bus.

Some termination manufacturers provide

automatically sensing terminators.

• External terminators should be attached to the

last external device in the SCSI chain.

Don’t use any other termination on the

external SCSI chain.

The last device on an internal SCSI chain

should also be terminated in one of several

ways.

Many Single-Ended Ultra SCSI and earlier

devices provide a jumper setting for applying

termination: place a jumper over the pins

designated for termination on the last device

on the internal cable. Check with your drive

manufacturer if you are not sure which pins

to use.

• LVD Ultra2 and Ultra160 SCSI devices cannot

supply their own termination.

Use an internal ribbon cable which has a

SCSI terminator attached to the end of it,

connect the unterminated end of the cable to

the host adapter card and the internal drives

to the subsequent connectors.

The terminator should be at the opposite end

of the cable from the host adapter card.

• Wide (16-bit) and Narrow (8-bit) devices can be

connected together on the same connector of

the host adapter card, but wide devices must be

attached first, followed by narrow devices.

To terminate the SCSI bus, the cable or

adapter used to convert from a wide (68-pin)

connector to a narrow (50-pin) connector

provides partial termination, allowing upper

8-bits (or byte) of the wide SCSI bus to be

properly terminated.

A narrow terminator should be used on the

last narrow device to terminate the rest of the

SCSI bus.

A SCSI bus without partial termination

between the wide and narrow devices may at

first appear to work correctly, but occasional

I/O errors occur without proper termination.

• If you use both internal and external connectors

and mix Single-Ended and LVD devices on the

same bus, even if using different connectors, the

host adapter card will operate with Single-Ended

signaling at UltraSCSI speeds.

• Automatic termination

When both internal and external connectors

are used, the host adapter card detects the

presence of devices and turns off

termination.

If devices are removed from one connector

of the card, the host adapter automatically

detects the change, and enables its own

termination.

• Software controlled termination

You may have to override the host adapter’s

automatic termination if only narrow devices

are attached to one connector and wide

devices are attached to the other connector

on the same bus.

The host adapter must supply partial

termination to the upper 8-bits (byte), but not

automatically.

Please refer to your ATTOI Utilities manual

for instructions on setting the host adapter’s

termination to Upper Byte.

• Termination power

Host adapters supply termination power to

the bus at all times and many SCSI devices

are also able to supply termination power.

SCSI signal quality, particularly with long or

marginal quality cables, may be improved if

the device supplies the termination power.

Contact your device manufacturer for more

information on your device’s ability to supply

termination power.

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