Pertec Interface Cable Length

[W2HX]

>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 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
>
>
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