Pertec Interface Cable Length

Chuck Guzis cclist at
Sat Aug 10 00:47:19 CDT 2019

On 8/9/19 9:05 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?

5 feet should be no problem--ISTR that Pertec specifies a maximum cable
length of 20 ft.  I use 10 foot cables routinely.

The Pertec interface is basically open-collector drivers into 220/330
ohms at the far end.

I have a Qalstar 1260, but it's the 1260S, the SCSI version--and I
rarely use it.

Looking at the schematics of the similar Qualstar 1052, the output
drivers are plain old LS240s; 24 ma totem-pole outputs.   The usual
practice is to use 7438s OC 48 ma outputs; it's certainly the case for
Pertec formatters.

So it could be that you're limited by the Qualstar design.   Twisted
flat cable might buy you some added distance or you could fabricate a
"repeater" with real OC drivers to put between the drive and the
controller to extend the length.


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