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

From: Pete Turnbull <>
Date: Wed Feb 16 13:27:28 2005

On Feb 16 2005, 13:52, 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?
> Looking at circuit diagrams for termination cards, it looks like I
> 220 ohm resistors, going to +5V.

Each bus line should have the equivalant of *two* resistors, normally
390 ohms to ground and 180 ohms to +5V, giving an equivalent of 123
ohms (nominal 120-ohm termination) and a no-load voltage near 3.5V.
 It's a standard resistor network package, but not a particularly
common one.

Some CPUs have 240-ohm (nominal equivalent) termination, but I don't
think that KDJ11 is one of them.

> How critical are the bus termination resistors?

Depends. We discussed that just recently. In a lightly-loaded system,
usually you can manage with just the set on the CPU. Which might be a
problem here!

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Wed Feb 16 2005 - 13:27:28 GMT

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