Ask THE SCSI EXPERT.

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

Q: When dual channel SCSI is integrated onto a motherboard, the manufacturer generally advertises the maximum speed at double the rate of each channel. For example, Dual channel Ultra2 SCSI at 80MB/sec per channel is advertised as having a maximum of 160MB/sec burst data transfer rate. If I were to purchase one of the newer Ultra160 SCSI drives, how would I physically make the connections to get the maximum 160MB/sec transfer rate? As I understand it, there would be two 68 pin connectors, one for each channel.

A: Ultra160 SCSI target devices only have one connector, but transfer data twice as fast. Many Ultral60 SCSI initiator devices (host adapters) have more Than one connector: To use the Ultra160 SCSI speed, the controller must be Ultra3/Ultra160. Ultra160 devices are backward compatible with Ultra2 controllers, but will run at the Ultra2 SCSI controller speed or 80MB/sec peak.

Note: the current media rate for drives' is around 20MB/sec. It requires four drives running long block transfers to use the bandwidth of the Ultra2 SCSI bus. For older drives, before the 36GB technology, the media rate was around 15MB/sec maximum.

Q: Is it possible for Low Voltage Differential (LVD) SCSI drives to be used on the same bus as Single Ended (SE) Ultra SCSI drives? If so, under what conditions? Is a single ended-to-differential converter required? It has been suggested to me by a sales person that LVD drives are "backward compatible" and "future-proof," but I'm not so sure.

A: That is correct; the LVD SCSI drives are really multimode and will work on both a single ended or LVD SCSI bus. There is no termination on the LVD SCSI devices, the termination must be provided in the cable system or by another device.

Q: We are using SCSI cables and terminators from Amphenol and SCA backplanes from Cremax. We are running at a rate of 80MB/sec (40MHz) LVD SCSI. Do we need to change our cables, terminators, and backplanes when switching to controllers and devices running at the rate of Ultra160 SCSI?

A: Your cables, terminators, and backplanes should all work fine for Ultra160 SCSI.

Q: Is it possible to test if termination is on or off on a single SCSI device (with ohm meter)?

A: Here are two simple tests that can be done on the termination with a power supply for termpwr and a meter.

Single Ended SCSI Termination

Apply 4.5 volts to Termpwr. Measure each of the signal pins. The voltage must be between 2.5 and 3.0 volts. Measure each of the signal pins. Current to ground should be 20 to 26mA. This determines the basic function of the terminator. There are many more tests that can determine if a terminator is functional, but these tests require more equipment.

LVD SCSI Termination

Apply 3.3 volts to Termpwr. Measure each of the signal pins. The voltage must be between 1.05 and 1.45 volts. The difference between the Plus and Minus pin for each signal pair should be 100 to 125mV. The Minus pin should be the Positive side; the terminator holds the bus negated.

Q: I have inherited a server, but due to poor historical record keeping I do not have specifications on the system. How can I determine the controller card interface and speed and the hard drive interface and speed?

A: Norton Utilities can determine both the devices and the operating speeds. You can find out some basic information under Windows operating system by going to the control panel, select system, select the device manager, and select SCSI controller and the Hard disks.