11 Aug
2019
11 Aug
'19
6:07 a.m.
Would folding the
excess cable up and covering
with anti-static plastic help?
unlikely. The typical problem is that the longer the cable, the higher the capacitance of
the transmission line. Therefore you get a lot of problems with rising and trailing edges
of the signals which can cause all kinds of problems if not terminated correctly. I agree
with the previous suggestion to make sure all termination is in place. If you can also
place a scope on one of the lines (with high impedance probe) you can see what the edges
look like.
________________________________________
From: cctech <cctech-bounces at classiccmp.org> on behalf of Douglas Taylor via
cctech <cctech at classiccmp.org>
Sent: Sunday, August 11, 2019 12:33 AM
To: Jon Elson; On-Topic Posts
Subject: Re: Pertec Interface Cable Length
On 8/10/2019 1:56 PM, Jon Elson wrote:
On 08/09/2019 11:05 PM, Douglas Taylor via cctech
wrote:
I haven't checked to see if there are terminators (Arnold the
Terminator) on either end. I did check the long cables for continuity
and found no problems. It may be an EMI problem. Would folding the
excess cable up and covering with anti-static plastic help?
Doug
I have a question about cable length - any
electrical engineers in
the house?
Connected a Qualstar 1260 tape drive to an Emulex TC02 qbus tape
controller in a pdp-11/53. The interface is pertec with 2 50 pin
cables.
When I use a pair of short flat ribbon cables, 18 and 30 inches each,
it works. Under RT11 I can INIT, Copy, DUMP, do a Directory.
It doesn't work when I use a pair of 5 foot long flat ribbon cables.
Are they too long? Do I need twisted pair type of cable? Is it
possibly a termination problem?
I have used cables about 20 feet long without trouble. The 2 50-pin
cables is the Pertec formatted interface, which is really forgiving.
Does you drive have terminators in both ends of the cable (both at the
TC02 end and the drive end)?
Now, I will mention that I have ONLY used twisted-pair ribbon cables
with both flavors of interface, never straight ribbon cable.
Jon