PC Hardware

Noise and SCSI

Any electrical signal other than data is called noise. Due to the many signals and electrical devices present, the interior of a computer is a noisy place. Computer manufacturers do many things to contain the noise inside the case, including adding shielding and grounding. Anything inside, or directly connected to, a computer is either a contributor to or a victim of the noise.

Because of the high data transfer speed, products using the SCSI-2 and later standards can be very sensitive to noise. Cables tend to act as antennae for noise. For this reason, proper cabling and minimizing of cable length are needed to maintain low noise in a SCSI system. Any noise spread through either the electrical power cables or the data cable is called "common-mode" noise.

A single-ended device communicates through only one wire per bit of information. This one wire is measured, or referenced, against the common ground provided by the metal chassis. Single-ended devices are vulnerable to common-mode noise (they have no way of telling the difference between valid data and noise). SCSI-1 devices are all single-ended.

Some SCSI-2 and SCSI-3 devices are differential-ended. These products employ two wires per bit of data-one wire for the data and one for the inverse of the data. The inverse signal takes the place of the ground wire in the single-ended cable. By taking the difference of the two signals, the device is able to reject common-mode noise in the data stream.

Under no circumstances should you try to connect single-ended and differential-ended devices on the same SCSI chain. You might fry the single-ended device and, if the differential-ended device lacks a security circuit to detect your mistake, you will probably smoke it as well.

Troubleshooting a Device Conflict

Determine which is the offending device by taking the following measures:

  • Load only the device drivers for the SCSI devices.
  • If the problem still occurs, use the F8 key to determine which driver conflicts. (Press F8 when starting MS-DOS or Windows 95 or 98-this will allow step-by-step confirmation of the startup process.)
  • If the device driver is an executable file, try running it with the "/?" option. This will usually show a variety of command-line switches for the device driver (for instance, MOUSE.EXE /?). For more details on MS-DOS commands programs and switches, see Tutorial 15, "Software: MS-DOS and Windows 3.x."

Here are some ways to correct the problem after you've found it:

  • Look in the manuals or "readme" files of both devices. The problem might be a common one with a known solution.
  • Try a variety of switches to see if any of them solves the problem.
  • Attempt to find an updated driver for one or both of the devices (the Internet is a good place to look).
  • If none of those solutions fixes the problem, you might be forced to choose between the devices or go to a multiple boot configuration.