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. FWIW, Chuck