Ask THE SCSI EXPERT.

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

Q: What is SAF-TE? Where can I get further information on it?

A: SAF-TE and SES are enclosure or device management. It allows you to monitor enclosure or device temperatures, voltage, current levels, fans, and LEDs; over the SCSI bus.

SAF-TE was the predecessor to the SCSI Enclosure Services (SES) ANSI standard. You can find SES on the T10 web site http:/www.t10.org/.

Q: What are the differences between IDE and SCSI for CD-RW drives? What does the buffer do and what are the benefits of it?

A: The buffer allows the full block to be transferred to the CD-R; if the full block does not get transferred there will be an under-run error. The earlier CD-R with small buffers received under-runs when any activity occurred on the system.

SCSI gives you less loading on the system over IDE during the write and will allow more activity on the system than IDE permits.

Q: What are the factors in deciding how to configure termination power on a SCSI bus? Is it harmful for more than one device to provide power?

A: Termpower is designed to allow multiple devices to provide Termpower. It must be provided by at least one device on the bus. There is diode or electronic isolation so that the device, with the highest voltage provides the current.

It does no harm for multiple devices to provide termpower. In fact, when using long SCSI cables, it is a good idea to have multiple devices. providing termpower, as it will help prevent voltage droop caused by IR drop in the cabling.

Each bus segment must have term power. Bus segments are created by SCSI Expanders which usually provide term power for the bus segments.

Q: Are the cables and terminators for Ultra 160 SCSI the same as for Ultra2 SCSI?

With an Ultra160 SCSI controller on my motherboard, should I be able to use Ultra2 SCSI hardware (such as cables, terminators and hard drives)?

A: Yes, the cables and terminators from Ultra2 SCSI can be used in Ultra160 SCSI. If you want the dynamic mode change in Ultra160 SCSI, the terminator should have the mode change delay extended to 100-400 milliseconds from two milliseconds. If this is a production product you should change the termination (either the delay or a new terminator with the digital delay built in). If this is a system you are using, power cycle if you switch bus modes from single ended to LVD SCSI.

The same disk drives and cables will work at the Ultra2 SCSI speeds. New Ultra160 SCSI drives will run at the faster speeds.

Q: What is the maximum impedance allowed on an external cable (with VHDCI connection at one end) if connected to an LVD SCSI bus?

A: The maximum differential impedance is 135 ohms. A loaded bus must stay above 85 ohms.

Q: What is the feature "Active Negation?" How does it work?

A: The SCSI bus started with passive negation, the terminators bias the bus to the negated state and the driver only asserts. For active negation, the drivers both assert and negate the bus for higher speed operation. There are still signals that required the passive negation when multiple drivers are on the same line, like BUSY, ATN, and RESET.