PDP 11/40 Unibus termination query...

Ethan Dicks ethan.dicks at gmail.com
Thu May 21 14:06:19 CDT 2009

On Thu, May 21, 2009 at 2:56 PM, Ian King <IanK at vulcan.com> wrote:
> Unless you have a DMA device - which is typically going to be something like a disk or tape controller - the NPG grant should be daisy-chained without break.


>  I disagree slightly with Ethan's assertion that this is something you'll play with these jumpers a lot: I've only done it once, for a tape controller.  Maybe I've just been lucky.  :-)  I think if I ever remove that tape controller and put in a non-NPR board, I'm going to find some sort of slide-on jumper for CA1-CB1, just in case....

It depends on how you use your machine.  In my experience, the
contents change.  If you don't have to remove a DMA-capable-board (to
borrow it for another machine, say), then you won't have to fiddle NPR
wires.  If you only have devices like an RX-11, it's not a DMA device
so you don't have to fiddle jumpers.  We used to move tape and disk
controllers several times a year as we mocked up customer
configurations to chase down bugs.  I'm glad we had a pile of
dual-height grant cards that were effectively free.  It saved us a lot
of time when we changed the machines, which we did from time to time.

If you only have one PDP-11 or you have enough peripheral cards for
each of the PDP-11s you do have then maybe you won't have to
reconfigure very often.



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