Pertec Interface Cable Length

Bill Gunshannon bill.gunshannon at
Sun Aug 11 08:59:00 CDT 2019

On 8/11/19 9:07 AM, W2HX via cctech wrote:
>> 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.

All of my Pertec tape drives (in the past) were located in
a differentrack from the actual computer so the cables were
always over 10' long and usually close to if not 20'.  Never
had a problem.  I seem to remember they were ribbon cables
with each odd/even pair twisted which probably meant one
active and one ground twisted together.

> ________________________________________
> From: cctech <cctech-bounces at> on behalf of Douglas Taylor via cctech <cctech at>
> 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?

I don't remember there being any specific termination like you see on
things like SCSI disks or RL Disks.

But then, it has been a long time since I had my last Pertec Tape
Drive.  Only 9-track I  have today is SCSI.



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