Re: your mail

Richard B. Johnson (
Sat, 8 Mar 1997 09:37:53 -0500 (EST)

On Fri, 7 Mar 1997, Edward Welbon wrote:

> On Mon, 3 Mar 1997, Richard B. Johnson wrote:
> > This is the SCSI driver. No external drives are attached, and the internal
> > drives are connected with a SHORT cable properly terminated on the far end.
> A too short SCSI cable is a no-no. The problem is that a scsi cable is
> actually a transmission line and the points where devices connects are
> essentially points of discontinuity which alter the characteristic
> impedance of the cable. Depending on the impedance of the the drives and
> the impedance of the cable, you can have *too* *short* a cable.


>From ANSI X3.131-1986, Page 16 paragraph 4.2, Cable Requirements...
"An ideal impedance match with cable terminators implies a cable
cable characteristic impedance or 132 ohms (single-ended option) or
122 ohms (differential option)....etc...etc" (I do this for a living).

Transmission line mismatches can cause problems when reflections from
impedance discontinuities interfere with the SCSI bus signals. This
interference can occur only with LONG cables. These problems can occur
at odd multiples of 1/4 wavelength at the highest frequency of interest.
This is the reason for the maximum allowed cable length of 25 meters.

Because of the attenuation of the high-frequency edges of the signals, the
maximum unterminated stub length allowed is 0.2 meters. Such stubs act as
capacitors and load the bus.

It is absolutely impossible for interconnections to be too short. In
Sun Boxes, the SCSI connections are made via very short PWB traces.

The Adaptec AHA-2940 is a very troublesome card. I have used Adaptec
AHA-1542 cards with a great deal of success. However, the 2940 will
not properly drive the bus. I first thought that my original board was
defective so I returned it to the vendor and got another one. Further
research has shown that this board is defective in design.

We replaced many of these boards with Buslogic BT-958 boards when we had
to drive many SCSI devices. If you are lucky, and use a very short cable,
you can run two Disk drives, a CDROM, and a tape drive, i.e., four
devices, with the 2940 board. The AHA-1542 could drive 7 devices with
long external cables. Its line transceivers were correctly implemented.
Those of the AHA-2940 are not.

Dick Johnson
Richard B. Johnson
Project Engineer
Analogic Corporation
Voice : (508) 977-3000 ext. 3754
Fax : (508) 532-6097
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