ask THE SCSI EXPERT.

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

Q: Will adding an UltraSCSI device to an Ultra160 SCSI controller slow down the Ultra160 SCSI devices or will it keep the highest transfer rate of each device? Also, we're using an Ultra160 SCSI RAID controller card with two channels for striping across two drives. Will we get better performance if each drive is on its own channel or does the speed remain the same, generally, if both drives are striped on one channel?

A: When plugging an UltraSCSI device into an Ultra160 SCSI configuration, the Ultra160 SCSI bus is forced into single-ended mode. Thus, the bus falls back to UltraSCSI speeds (20MB/sec for narrow, 40MB/sec for wide). In order for the configuration to run at Ultra160 SCSI speeds, there can be no single-ended devices (i.e., SCSI or UltraSCSI) attached to the bus.

The UltraSCSI device should be on either a separate bus or placed after a LVD SCSI-to-single ended expander. Both will keep the Ultra160 SCSI device running at its higher performance.

Using two buses will not increase the performance until you have more than four drives on a bus. The Ultra160 buses are 160MB/sec, the current generation of 7200 RPM disk drives is 27MB/sec sustained and the 10K RPM drives are 36MB/sec sustained. It takes a minimum of four 10K RPM drives to saturate a bus doing long block transfers.

Q: Is there a difference between differential and non-differential cables?

A: Speaking from the SCSI specifications point of view, there are some differences between single ended and differential cables related to the assignment of SCSI signal pairs to twisted pairs in the cable itself. This difference can be mitigated by careful selection of wire pairs. It is very important for the REQ and ACK signals to be in the very center or core of the cable for maximum isolation from external radiation (noise). The control lines should be in the next layer of pairs and the data lines should be in the outside layer of pairs.

Another potential problem is that some SCSI cable manufacturers save manufacturing costs by eliminating many or most ground wires and "commoning" them at the connectors. Using common ground wires reduces noise rejection, increases crosstalk, and changes cable impedance. In spite of not meeting SCSI specifications, the cable may work for short, single ended, Slow SCSI cables, but it absolutely will not work for LVD or HVD differential SCSI cables. These effects can cause SCSI problems that are extremely difficult to troubleshoot, especially for longer SCSI cable lengths and for anything faster than Slow SCSI.

To be on the safe side, buy a SCSI cable that is termed a "differential SCSI cable," rather than a "single ended SCSI cable." SCSI cables are not the place to try to save money in your SCSI system. Buy quality cables that are properly assembled.