Pertec Interface Cable Length

[Douglas Taylor via cctalk]

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?

Doug