Ask THE SCSI EXPERT - Technology Information

Computer Technology Review, June, 2001

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

Q: What is the difference between a single ended device and a differential device?

A: Single ended has a negative signal line and a ground return for each signal in a SCSI cable. The normal drive voltage asserted is 0.5 volts and negated is 2.85 volts.

Differential, SCSI-2 version, which is now HVD (High Voltage Differential), is obsolete in SPI-3. It is based on EIA 485 with typically a 2 volt signal between the differential pair of wires. It is not the same pinout as Low Voltage Differential or single ended--a ground is shifted to the outside of the connector.

Differential, SPI-2, SPI-3, and SPI-4 LVD SCSI are low voltage differential. This is based on the single ended pinouts. Most devices are multimode allowing them to run in either single ended or low voltage differential. A typical LVD signal is 500 mV.

Q: Please define the term "single ended."

A: Single ended is the electrical interface. Only one line is driven, versus differential when a positive and negative line are driven. The electrical interface for single ended relies on the terminator for pull up (2.5 to 3.0 volts) and termination of the line. The driver asserts or turns on to drive the line low to 0.0 to 0.5 volts depending on the technology (Bipolar or CMOS).

The normal SCSI single ended interface has a signal return ground for each single line reducing ground bounce.

Q: I would like to know more about differential SCSI. I have a differential SCSI hard drive, an UltraSCSI hard drive, and a narrow SCSI hard drive. Would it be possible to put these three drives on the same chain and controller card? Will the slow drive impede the performance of the faster drives?

A: The old SCSI- 2/SCSI-3 differential drive is not compatible with any interface, as it has a different pinout and different electrical characteristics. It requires an expander differential-to-single ended to work. The expanders are about $350, which normally makes it cheaper to buy a new drive.

The LVD SCSI (Low Voltage Differential) used in Ultra2 SCSI, Ultral60 SCSI, and Ultra320 SCSI is compatible with single ended, and will switch to single ended mode working at the UltraSCSI speeds.

The wide and narrow drive will work on the same bus, but make sure the high byte is terminated at the transition point to narrow.

Q: Does a differential SCSI host adapter support the ANSI SCSI standard?

A: There have been three different electrical standards over the years for SCSI. Those three are single ended, differential (now called HVD SCSI) and LVD SCSI (Low Voltage Differential). Ultra2 SCSI and later versions are only LYD SCSI; it is the only interface that will work with the higher speeds demanded by servers and high end workstations. HVD SCSI is documented in SCSI-2 and SCSI Parallel Interface-2 (SPI-2). LVD SCSI is documented in SPI-2, SPI-3, and SPI-4.

Q: I am writing firmware for a SCSI target device and I am looking for some documentation to help me. I have a copy of the SCSI-2 standard. I am specifically interested in the sense key and sense codes. Are there any standards that must be followed when returning sense data? For example, if my target experiences a read error, am I required to return specific sense data, or do I just need to return sense data that the host understands?

I have the source code for the driver on the host system.

Finally, are there any books or other documentation that you could recommend that cover target firmware?

A: SCSI-2 is available Global Engineering or the draft copy on the SCSI committee (T10) web site: www.t10.org.

The SCSI encyclopedia from ENDL is a complete reference set.