ask THE SCSI EXPERT.

Q: Is it possible to use an Ultra2 wide hard disk with an Adaptec 2940 AU? If so, how?

A: The Ultra2 drive will ended automatically when it is attached to a single-ended bus. The Ultra2 drives do not terminate the bus.

Adaptec AHA2940U is a narrow controller, which will require an adapter 68 pin to 50 pin. The high byte must be pulled up or terminated. Adaptec AHA2849UW is a wide controller. Use active terminators at the end of the bus and you won't have any problems.

Q: What is SCSI fairness?

A: Fairness is in the annex of SPI-2 and will be included in Packetized SCSI commands defined in SPI-3. Fairness by a device means that it allows other devices means that it allows other devices to access the bus. If it is a high priority device on the bus and it is very fast, it will take control of the bus every time the bus goes bus free. When fairness is implemented, it will not try to get the bus for a number of bus arbitrations, even if it has a task to perform on the bus. This allows lower priority devices a chance to use the bus.

This became a very obvious need with RAID striping, when the highest priority device transferred all of its strip information. This tied up the controller, which could not process the message because it could not get the other stripes of the message from the lower priority devices.

Q: We are currently using SCSI cables with active termination. Some of these cables are with twisted wires between the connectors and some of them are straighforward cables. Could an untwisted cable be used for connecting LVD drives to a LVD host-adapter or do you need to have the twisted cable?

A: The flat cable for LVD SCSI introduces more crosstalk, while one or two drives with a short cable works well generally. The twisted flat cable reduces the crosstalk, which allows for more drives and longer cables. I would strongly recommend the twisted flat cable.

Q: Are SCSI devices backwards compatible? For example, will an Ultra2 Wide drive work with a basic narrow controller?

A: If your Ultra2 device is "Multimode LVD" and your existing SCSI host adapter controller card is single-ended, the Ultra2 device will be compatible. It will, however, run in the single-ended mode and will be limited to the single-ended data throughput and cable lenghts.

The disk drive will not have termination; termination must be provided by another device or in the cable system. If the drive is being adapted to a narrow bus, the adapter must pull up or terminate the high byte. If you keep the above in mind, you will have no problems installing a wide peripheral on a narrow SCSI bus, as long as your cabling and terminations are correct.

Q: Are SCSI devices backward compatible with controller cards? Can I use a Fast Wide hard drive (68 pin) with a SCSI-2 (50 pin). Controller card? I know that I can get a cable that will go from 50 to 68 pins, but is this dependent upon the device, controller, or both?

A: Yes, SCSI controllers and peripherals are backward compatible as long as they both are single-ended or both are differential. You can connect your Wide HDD to your narrow controller with your 50 to 68 pin cable. The device will operate at the maximum speed that both the controller (Initiator) and the device can negotiate. Other devices on the bus do not restrict speed. You can connect single ended devices to differential controllers for vice versa) with an electronic product called a SCSI Expander, specifically a SCSI Converter.

The latest "multimode LVD" SCSI devices are backward compatible with older devices only through the single-ended interface. Again. LVD and HVD devices can be connected together with a suitable SCSI Expander.