PDP-11/73 booting! - test-drb@ccmp.vtda.org

12 Dec 2005

...

Subject: Re: PDP-11/73 booting!
   From: Pete Turnbull <pete at dunnington.plus.com>
   Date: Mon, 12 Dec 2005 20:41:06 +0000 (GMT)
     To: "General Discussion: On-Topic and Off-Topic Posts"<cctalk at
classiccmp.org>

On Dec 12 2005, 15:03, Allison wrote:

  The BA23 came with a microPDP-11 backplane or one
for the uVAX-II.
 While the differences are small when used with care they are differnt.
  For the PDP-11 use the MicroPDP11 backplane is
desired as the uVAX
 one has three slots that are CD wired for PMI bussed memory.
 the DISK interface and distribution board in the back of the drive
 cavity however are the same. 
All the ones I've ever seen in the field are the same regardless of
whther for microPDP-11 or microVAX.  I can't see why the microVAX one
would be different; the microPDP has the CD slots for PMI but the
microVAX doesn't care -- it uses over-the-top connections for the
memory.

The BA23 backplane for a microPDP-11 is an H9278-A and does indeed have
three slots Q22-CD followed by five serpentine Q22-Q22.  That's the
same as my microVAX.  And that agrees with the Tech Manuals I have. 
Ah, but it doesnt agree with mine. You can use the H9278 for Qbus -11
but if you want to maximize the slots there was a differnt early option
BA23 that had no CD slots(all were AB serpentine) as none of the 
processors before the PMI equipped 11/73 used PMI.  Early microPDP-11s
wer really 11/23Bs.  As a result there really were two similar but 
not exactly the same flavors. I know the gory details as my BA23 uVAX
was hacked from the uPDP-11 and I needed a differnt backplane 
(memory compatability) and the front pannel insert for the drive 
control was different.  At the time I was a Digit so getting the 
pieces from the systems lab was easy.

Allison