Much swearing - was Re: Do-It-Yourself PDP-11 help!

From: Steven Canning <cannings_at_earthlink.net>
Date: Wed Feb 16 17:20:44 2005

On Wed, 16 Feb 2005, Gordon JC Pearce wrote:

> On closer inspection, the termination resistor pack beside the "B"
> connector on my KDJ11-A is cracked. How critical are the termination
> resistors, and what value resistor pack do I need?

Not entirely as I've learned, though they might be in your case. Also,
there might be an imbalance in the bus because some signals are not being
terminated? I'm speaking entirely out of ignorance here, so smite me if
I'm wrong.

Sellam Ismail


I'll "smite" you but only a little bit... The importance of termination
resistors depends on how much of a transmission line the cabling "appears"
to be and how high the frequencies involved are ( a good "squarewave" needs
bandwidth ten times the fundamental to be nice and square, so a 1 megahertz
squarewave needs 10 megahertz of bandwidth). This is because the squarewave
is composed of "odd" harmonics and you need a good 3rd, 5th, 7th and 9th
harmonic to look square. Also the termination resistors would want to be at
the "end" of the run or at the last device. Their purpose is to "match" the
impedance of the load to the driving entity ( driver chips output stage
plus cabling ) so as to prevent " reflections " of the signal from feeding
back into the line. The two antagonists to squarewaves are impedance
mis-match and line capacitance. This is probably way more than you wanted to
know, but hey it's already out there.....

Best, Steven Canning
Received on Wed Feb 16 2005 - 17:20:44 GMT