Ask THE SCSI EXPERT.

Visit the SCSI Trade Association's web site (www.scsita.org) to ask the SCSI Expert a question.

Q: I need a source for an ISA SCSI card where specific IRQ/DMA/I/O required by B/U software ("Fastape") allows for 5,3,330h in a 486 PC running an ARCHIVE 2150eS drive.

A: The new ISA cards are jumperless, plug and play cards. These are set up to unused IRQ, unused I/O, and Memory space.

An old SCSI card should fit your requirements. Look at used computer options, for example, the Adaptec 1540 or Future Domain cards.

Q: In a recent situation, I disagreed with a colleague on what I believed to be a known fact about SCSI and its performance benefits. The remark was that having a CD-ROM hooked together on the same SCSI bus as an Ultra Wide (40MB/sec) drive would cause the drive to operate only at the lower rate of the CD-ROM (approx. 5MB/sec). I argued that this was one of the benefits of SCSI, that it could differentiate between the Ultra Wide and slower SCSI peripheral on the same bus and not reduce the performance of the drive. Could you please answer this question for me and point me to any Web sites acknowledging the answer for verification?

A: You are correct. The only time it will affect the disk drive is while a transfer is occurring with the CD-ROM drive. If you put a CD-ROM drive on an Ultra2 SCSI bus, it will switch the bus back to single ended, Ultra Wide performance, but not back to 5 meg Async.

Q: Is there any way of having two hosts talk to one drive? Is there a controller that does not reserve the drive or always releases it? Is there a Y cable that would let you connect without requiring the other controller?

A: Two controllers can be on the same bus with different IDs and talk to the same device. They must use the Reservation and persistent reservation. See the definitions in SPC-2. There are multiported drives specifically for the applications. Do not use a Y cable.

Q: What is the difference between a Passive and Active SCSI Terminator? How do I know which one I need?

A: Passive termination is a resistor stack: Termpwr (4.0 to 5.25 Volts) 220 ohm resistor to the signal line and 330 ohms from the signal line to ground. This is not a good impedance match and the pull up current changes with the termpwr voltage. Active termination uses a regulator and an 110 ohm resistor; this is a better impedance match and significantly better pull up current.

These are both single ended SCSI terminators. Ultra2 and beyond uses LVD SCSI (Low Voltage Differential) which requires a multimode or LVD only terminator. Active termination is required for Ultra SCSI and recommended of all SCSI buses.

If you are running Ultra SCSI or several devices on the SCSI bus, you should be using Active termination. Bus errors and random bus hangs will occur with passive terminations with large configurations or high speed.