Ask The Scsi Expert

Computer Technology Review,  May, 2000  

• E-mail
• Print
• Link

Q: What is the difference between termination power and termination? My understanding is that terminators are resistor packs that "close off" either end of a SCSI chain and that termination power is merely power for the terminator and may be supplied by any device on the chain.

I am told that setting the termination power jumper on a device enables termination for that device. However, the installation manual for the device stresses not to remove the jumper.

A: At one time, SCSI termination was resistor packs, but that was about ten years ago. Active terminators have been used for single ended for about ten

years and LVD SCSI termination is a combination of regulators and termination with current mirrors.

Termpower provides power to the terminators. There should be an enable termination jumper.

Q: How can you tell the difference between single ended and differential devices if they are not labeled? I have a customer with several tape libraries manufactured by the same company. They know that one of the tape libraries is differential, but none of them are labeled. They want them connected, but we cannot determine which one is differential.

A: Here is a procedure for determining whether the SCSI device is single ended or differential and, then, for distinguishing an HVD from an LVD device: If you have the model number and manufacturer of the device, take a look at the manufacturer's website. It may be a current model. Also, some manufacturers have historical information on products that they are no longer manufacturing.

If you cannot find any information on the device, you can try the following:

To determine if the interface is single-ended or differential requires the use of a multimeter that measures low resistance values. Using a multimeter that is capable of measuring low values of resistance, measure the resistance between pins 7 and 9. These two pins work regardless of the kind of SCSI connector. However, remember that while the pins on most SCSI connectors are numbered 1,2,3, [ldots] along the top row of pins and, then, continued along the bottom row, the IDC connector used for 50-pin internal ribbon cable connections have odd numbered pins in one row and even numbered pins in the other row. If this resistance value is close to zero ([less than] a few tenths of an ohm) the interface is single ended. If the value is 0.5ohms or more, it is differential--either HVD or LVD.

To determine if the device is HVD or LVD, the best way is to power the device and measure the voltage from the DIFESENSE pin to a GND pin. If the voltage value is less than 0.5VDC, the device is single-ended; if the value is between 0.7V and 1.9V, it is LVD; and if the value is more than 2.4VDC, the device is HVD. When you are connecting voltage to the units, be absolutely sure you have the correct voltage values and the correct polarity or you will fry the device.

COPYRIGHT 2000 West World Productions, Inc.
COPYRIGHT 2000 Gale Group