9 Aug
2019
9 Aug
'19
10:47 p.m.
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