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-boun...@classiccmp.org> on behalf of Douglas Taylor via > cctech <cct...@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? 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. bill ' bill
