How long can my SCSI bus be?

How long can my SCSI bus be?
The SCSI bus length limits are based on the speed of the fastest device
attached to the bus.

Excess cable length is also a bad thing, so basically all these factors
must traded off against each other to build the best SCSI cable for a given
situation.

Here’s a table which shows the limits:

Speed of Max Single-Ended Max HV Diff. Max LVD bus
FASTEST device bus length bus len. length
============== ================ ============ ============

5 MHz 6 meters 25 meters 12 meters
(SCSI1 synch.)

10 MHz 3 meters 25 meters 12 meters
(SCSI2 FAST) (Note 1)

20 MHz 1.5 meters 25 meters 12 meters
(Ultra or (Note 2)

Fast20)

40 MHz Not recommended 12 meters 12 meters
(Ultra2 or
Fast40)

Note 1: not recommended in SCSI-2 spec.
Note 2: 1.5 meters is my recommendation. The SCSI-3 SPI spec.
gives a much more complicated recommendation.

These limits assume the use of good quality cable, and the use of active
terminators or LVD/SE terminators at each end of the bus.

Notice that I used the term MHz to specify speed since MB/sec. changes with the bus width.

Note: Bus width doesn’t change the maximum allowable length. The bus width is independent
of bus length or speed.

The above table assumes that you know the max. speed of your devices (usually by looking in
the manuals). Some software (like Adaptec EZ-SCSI) provides a driver status monitor which
will tell you what mode the devices are actually in. This is important, since any synchronous
speed must be negotiated by either the device, or the adapter. The speed actually used
will be the least common denominator between the two.

For example, if a Fast20 disk is attached to a "SCSI2" host adapter that
only goes up to Fast10, the device will only run at 10 MHz.

In systems with high performance disks and external peripherals which
require long cables (i.e. external scanners, tapes or CDROM changers), you
may want to put the external devices on their own bus to avoid having to
slow down the fast disks. There are dual channel host adapters to make
this simpler (avoids using multiple IRQs etc).

The SCSI Trade Association also has a handy table at:
http://www.scsita.org/aboutscsi/

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