>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. ________________________________________ 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? Doug
