Length of Cables for ESDI Hard Disk Drives

From: Richard Erlacher <edick_at_idcomm.com>
Date: Thu Aug 2 11:57:39 2001

You shouldn't be having a cable length problem with the ESDI drives. The fast
signals on the data cables (1 per drive) are driven with differential drivers
capable of considerable length at that bandwidth. In fact, the number 40 feet,
sticks in my recollection. The signals on the 34-conductor cable aren't
particularly fast, but aren't driven, typically, with drivers capable of what
the diff drivers can do. I doubt you use that much cable, that you'd see the
impact of cable length. If you really think it's the cable, I'd recommend
switching to a twisted pair ribbon from the typical flat ribbon. It won't help
much, and what you might try is simply using a different cable. They do fail
sometimes.

Dick

----- Original Message -----
From: "Jerome Fine" <jhfine_at_idirect.com>
Newsgroups: alt.sys.pdp11,vmsnet.pdp-11
To: <classiccmp_at_classiccmp.org>
Sent: Thursday, August 02, 2001 8:36 AM
Subject: Length of Cables for ESDI Hard Disk Drives


> I have had a problem with some of my ESDI disk drives for a number of years
> which may have a very simple solution.
>
> When I use the drives in a BA123 box with an 11/73, the system functions
correctly
> while I am using 7 quad boards or less. Whenever I add the extra boards, the
READ
> operations on the Hitachi ESDI DK515-78 disk drives (Sigma RQD11-EC quad ESDI
> controller) take about twice as long. The WRITE operations are still just as
fast.
>
> I have been looking at the problem again recently and while it has not yet
been resolved,
> I now begin to suspect that the cables between the RQD11-EC controller and the
drives
> may be too long. Just a guess is that the total length of the 34-pin cable is
six feet (with
> the first portion to the first header being four feet and three more pieces of
eight inches
> making a total of six feet to the last header) and the 20-pin cables (there
are four with
> three currently in use) are about four feet each. In addition, the three ESDI
drives currently
> in use run off their own PC power supply (in fact it is the actual power
supply that came
> with the drives within a large raid set-up to be used with a VAX). I also
salvaged the fans
> inside the box and each drive has its own fan. Normally, I only turn on one
ESDI drive
> when the other ESDI drives are not being used for backup purposes. In
addition, I
> have installed a heavy ground cable between the PC power supply and the BA123
box.
> In case anyone asks, the last drive on the 34-pin cable is correctly
terminated and
> the other drives are not terminated.
>
> Since everything runs VERY well in the BA123 with the DK515-78 drives while I
> use seven or less quad boards, I have not tried to investigate much until
recently since
> the extra boards are used so rarely it has not been a problem 99% of the time.
>
> And the fact that everything works so well has not led me to suspect the
length of the
> 34-pin/20-pin cables. Does anyone have any suggested maximum cable lengths?
>
> Sincerely yours,
>
> Jerome Fine
>
>
Received on Thu Aug 02 2001 - 11:57:39 BST