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

[Pete Turnbull]

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
want
> 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