chain of devices. Some SCSI devices are self-terminating, but if yours isn’t, you will need a separate terminator. Older SCSI-1 devices used simple passive termination, which uses resistors to stop the signal from reflecting back onto the bus. Faster speed single-ended devices also use active termination, which adds voltage regulators to the resistors to more efficiently terminate signals.

SCSI-2 and SCSI-3 devices need terminators based on the type of signaling being used. LVD (Low voltage differential), HVD (High voltage differential), and Singleended devices need a terminator for each specific type.

SCSI device configuration

When configuring a SCSI bus, you must be aware of several rules and limitations. These include host adapter settings, SCSI ID setting, and mixing different SCSI types.

Know how to configure a SCSI bus with a host adapter and devices; pay careful attention to IDs, cable lengths, and termination.

Host adapters

The SCSI host adapter facilitates communications between the system bus and the devices on the SCSI chain. The host adapter must be compatible with the SCSI devices that you are using. Most host adapters are backwards-compatible, so older devices can still be used with your system. The host adapter must also be electrically compatible with your devices. You can’t mix HVD types of devices on the same bus with single-ended or LVD type devices. Single-ended and LVD devices are electrically compatible, but your devices must support multimode operations. The only limitation is that the bus will only run as fast as your slowest device. Newer host adapters don’t have this speed limitation, because they use special segments on the same SCSI channel to separate the devices, or sometimes run a separate SCSI channel for devices running on a different mode.

Devices configuration

The SCSI system is configured as a bus topology because devices are chained together from one end to the other and terminated at each end. Original 8-bit SCSI devices can have up to 8 devices on the bus. The wide 16-bit versions can have up to 16 devices.

Device IDs

Each device on a SCSI bus must have its own separate identification number. This is how the system allocates control of the bus to a specific device. The numbers also give a priority level for the devices; higher device IDs have greater priority than lower numbers. This is why the SCSI host adapter is usually designated as device 7.

To set the device ID, older SCSI devices use jumpers on the card to configure the ID. Newer devices use a small button that, when pressed, can cycle through the choices of ID numbers. SCSI IDs can also be regulated through software.