Pertec Interface Cable Length

Douglas Taylor dj.taylor4 at
Sat Aug 10 11:45:25 CDT 2019

On 8/10/2019 4:57 AM, Dave Wade wrote:
>> -----Original Message-----
>> From: cctech <cctech-bounces at> On Behalf Of Douglas Taylor
>> via cctech
>> Sent: 10 August 2019 05:06
>> To: General Discussion: On-Topic Posts <cctech at>
>> Subject: Pertec Interface Cable Length
>> I have a question about cable length - any electrical engineers in the house?
> Its electronics, rather than electrical engineering. Electrical Engineering is power distribution.
>> 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.
> So the hardware is good.
>> 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 can't see 5 foot being too long for data from a tape, the data rates aren't huge. At most you have added 10ns to the delay times.
> On the other hand I have been wrong in the past and could be wrong again..
> I assume you have checked the cables. Ribbon cables are prone to come loose from the IDC pins if it’s a IDC connector, and if soldered can break....
>> Doug
> Dave
I bought the long cables off ebay, so they have to be good? Right? I 
think the short cables came from a hamfest.

The cables can be fairly long, I remember interfacing a TU80 to an 
Emulex QT14 (maybe) and the DEC cables were round and about 15 feet 
long.  And it worked.

It was too late last night to begin checking the long cables for 
continuity, so I fired off the email instead thinking it may be a 
termination problem.

Is it possible for the IDC and Card edge connectors to be put on wrong?  
You would want pin 1 to map to pin 1, and so on.


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