[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces
and Configuration | Small Computer Systems Interface (SCSI) | SCSI Configuration ]
Number of Devices
One of SCSI's strengths as an interface is its support for large numbers of devices on
the bus, and the fact that different "conversations" between devices can be
taking place simultaneously. SCSI buses can support as many as 16 different devices;
compared to the IDE/ATA interface and its limit of two,
this offers significant flexibility.
There are four different issues that influence the number of devices that can be used
on a single SCSI channel:
- Bus Width: SCSI was originally defined as a narrow,
8-bit bus, with support for 8 different devices. When wide 16-bit SCSI was created, this
was expanded to support for 16 devices. Since the host adapter is itself a SCSI device,
this means a theoretical maximum of 7 other devices for narrow SCSI, or 15 for wide.
- Transfer Mode and Signaling Method: Certain transfer
modes and signaling methods limit the theoretical maximum number of devices, due to
electrical signal integrity considerations. This is particularly true of Wide Ultra SCSI, where single-ended implementations are
limited to 8 devices despite being 16 bits wide, because single-ended signaling can't
handle 16 devices on a cable at that speed.
- Cable Length: Many SCSI transfer modes involve a "tradeoff"
between cable length and the number of devices that can be supported. For Ultra SCSI in
particular, reducing the number of devices on the chain allows the use of longer cables
and vice-versa. Newer implementations that use LVD signaling
are normally limited to 12m in length, but this can be extended to 25m if only two devices
are used on the cable.
- Practical Considerations: The limitations above are all based on
theoretical considerations. There are also practical issues involved in running many
devices on a SCSI bus. For example, you may be able to run 16 hard disks on your SCSI bus in
theory, but to do this would require a very large system case and a very power power
supply. For external devices there are similar issues to be addressed. Long cables with
many connectors are also very expensive and may require devices to be placed close to each
other so the cable will reach all the devices. Having many devices can make high
reliability more difficult. And so on...
This table shows a summary of all the different transfer
modes and signaling methods, and the number of devices supported for each.
 Note: It's pretty unusual
in most systems to need more than 8 devices, much less 16. If this is an issue, however,
you can connect more devices either by using two host adapters in the PC, or a single host
adapter that supports multiple independent channels.
 Next: SCSI Device IDs
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