DIGITAL Personal Workstation a-Series systems that include a SCSI bus currently use the Adaptec AHA-2940UW Ultra Wide SCSI PCI host adapter. At this time, DIGITAL supports up to four drives attached to the adapter's wide internal bus. Narrow devices can be attached to that bus when a 68- to 50-pin adapter is used. At this time, no external devices are supported on the same adapter used for internal devices. Add another SCSI adapter if you need to attach external SCSI devices.
For more SCSI configuration information, see:

• Configuring the SCSI Adapter
• SCSI IDs
• SCSI Termination
• SCSI Length
• Glossary of SCSI Terms

Configuring the SCSI Host Adapter

The AHA-2940UW adapter is configured using the SCSISelect utility. The utility can be started from the AlphaBIOS firmware when the following prompt appears during the initialization of AlphaBIOS:

Once the utility is running, use the up and down arrow keys to select SCSISelect menu options. Press [ESC] to return to the previous menu. Restore the default settings by pressing [F6]. To exit SCSISelect, press [ESC] until a message prompts you to exit. If you changed any settings, you are prompted to save the changes before exiting. Reboot the system for the changes to take effect.

Configuring SCSI IDs

The DIGITAL Personal Workstation a Series currently supports up to four internal SCSI devices. The adapter can support SCSI IDs ranging from 0 to 15. Each SCSI device must have a unique SCSI ID. Typically, the host adapter is ID 7.

Note: The CD-ROM drive is an ATAPI device attached to the IDE port.

SCSI Termination

Termination on the AHA-2940UW host adapter is controlled by software commands through an adapter utility. The default setting is Automatic. In Automatic mode, the adapter detects attached cable(s) to either one or two of its three connectors (cables on all three connectors is an illegal configuration). The adapter then sets termination accordingly, as show in the following table:
Note: You also can make these settings manually using the SCSISelect utility.

No internal SCSI devices should be terminated. (Termination must be disabled if a device is in the middle of the bus.) The internal SCSI bus is terminated by the SCSI controller card at one end, and a SCSI terminator plug on the cable at the other end.

There are several ways to disable termination on SCSI devices, including removable resistor packs, switch settings, or a terminator plug. Refer to the device documentation for details.

SCSI Bus Length

For reliable operation of UltraSCSI devices, do not let the bus length exceed 1.5 meters (4.9 feet).

Glossary of SCSI Terms

Asynchronous SCSI

A method of sending data over the SCSI bus. The initiator sends a command or data over the bus and then waits for a reply (REQ/ACK handshake).

Differential

Uses two wires to drive one signal. This reduces common mode signal noise and lets the SCSI bus run faster with more length and reliability. It is electrically incompatible with single-ended devices.

Fast SCSI

Allows faster synchronous operation on the bus (10 MBsec instead of 5 MBsec; up to 40 MBsec when combined with Wide SCSI).

Narrow SCSI

A term atrributed to 8-bit SCSI devices.

Single-ended

"Normal" electrical signals. Uses open connector to the SCSI bus.

Synchronous SCSI

Rather than waiting for a reply (ACK), devices that support synchronous SCSI can send multiple commands over the bus.

Terminator (active)

Uses one or more voltage regulators to provide a more accurate termination voltage than a passive teminator, which uses resistor voltage dividers.

Termination (passive)

A group of resistors on the physical end of a single-ended SCSI bus that dampens signals on the bus. Each terminated signal is connected by 220 ohms to +5V (TERMPWR) and 330 ohms to ground.

UltraSCSI

Allows up to 20 megatransfer timing on the bus.

Wide SCSI

Uses a 68-pin cable to send data 16-bits wide (instead of 8 bits).