ask THE SCSI EXPERT.

Q: Can my Fast SCSI (narrow) adapter work with Ultra SCSI (narrow) HDD?

A: Yes, but it will be limited to the Fast SCSI speed. SCSI works at the fastest rate at which the controller and device will operate and the negotiation is with each device.

Q: Where can I get SCSI documentation and specifications?

A: The best source for SCSI documentation is the T10 SCSI Committee website. It is at www.t10.org. You can download all the SCSI-3 specifications that are still in committee. If you need Approved Standards, like SCSI-2 or SCSI SPI-2 specifications, you can purchase them from Global Engineering or ANSI. You will find their information also on the T10 committee website.

Q: When I connect wide SCSI to narrow SCSI, should I use the high eight bits or the low eight bits? If I use the low eight bits, what can I do with the high eight bits and vice versa?

A: The low eight bits and parity are used and the high byte must be pulled up or terminated. If you are attaching a narrow device to a wide SCSI bus, you must use the "lower byte." The reason is that the narrow device uses the lower byte only. Narrow devices do not use the high (upper) byte. Only wide SCSI devices use the upper byte. If you follow the standard cabling scheme for SCSI, you do not have to worry about this "problem."

Q: I am working with a distributed control system that uses a SCSI interface. The distance between two SCSI connections is going to be about 30 feet. I will need a SCSI repeater. Where should the repeater be installed?

A: There are three electrical interfaces for SCSI. Single Ended SCSI HVD or differential SCSI bus length is 25 meters (81 feet) with up to 16 SCSI Devices. LVD SCSI 12 meters (40 feet), 16 devices and 25 meters point to point. Single-ended is limited six meters (20 feet) to 1.5 meters (five feet), depending on the speed and the number of devices.

For single ended, a single SCSI repeater may not be adequate for your needs, depending on the speed of your SCSI bus. If you have Slow SCSI (5MB/sec maximum for narrow) one repeater can double the bus to a max total cable length of 40 feet. If you are using Fast SCSI (5MB/sec to 10MB/sec maximum for narrow), one repeater can double the bus to a maximum total of 20 feet. Finally, for UltraSCSI (10MB/sec to 20MB/sec maximum for narrow), one repeater can double the bus to a maximum total of ten feet.

You can see that if you have Fast or UltraSCSI, you will have to use two repeaters back to back. As usual, in any cabled system, it is best to keep the cable lengths as short as practical. Also, be sure to use quality, correctly assembled SCSI cables to avoid problems with cabling.

Q: Why are SCSI buses limited in length?

A: For single ended, a loaded bus requires the reflected wave to insure that the signal is high enough. Bus lengths are limited for the round trip time of the signal on the bus. The faster the bus is running, the shorter the bus.

In situations using LVD SCSI, it is the loss, high frequency attenuation that generally limits the bus to 12 meters on a loaded bus, (up to 16 SCSI devices). However, in point to point configurations, the bus can run up to 25 meters.