DSD-440 and/or drive recommendation for PDP-8 - test-drb@ccmp.vtda.org

20 Jun 2005

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Subject: Re: DSD-440 and/or drive recommendation for PDP-8
   From: "vrs" <vrs at msn.com>
   Date: Sun, 19 Jun 2005 21:45:19 -0700
     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>

From: "Allison" <ajp166 at bellatlantic.net>
  From: "vrs" <vrs at msn.com>
 (It's easily doable, but not trivial.) 
 Heres the trick.  Don't copy the parts, copy the logic and use
 standard TTL.  Most all the functions are easily available or
 or sometimes available in denser (newer relatively) parts.
 Trying to copy verbatim will be hard unless you have a lot of
 DEC junker cards to strip.  However most of the logical functions
 are simple so substitutions are not as scary. 
Heh, I did that, but I shot myself in the foot by using the denser parts and
not quite getting it right.

Mine is actually a Posibus card (Ethan has the other prototype).  There are
other problems with Omnibus cards, too.  You can get the bus interface close
to spec., but not dead on, AFAIK with modern parts.  Posibus is more
forgiving.  (Though you can get close enough for small Omnibus backplanes
pretty easily.)

    Vince 
I'll limit myself to Omnibus, experience there.  Posibus and older
machines I've only done programming on.

It's not as dire as you'd expect. There are a few characteristicts that
must be met.  Input current, lower is ok and newer devices are good 
there. CMOS as inputs must be used with care as they have to switch 
with a postive swing of under 3V(omnibus, any TTL/DTL interface.).
The next is output drive, most newer devices exceed those.  Last is
output switching characteristics such as Vol at load current, again 
thats easily met wit newer devices. What may be a bit worrisome is
that new devies sink harder on the bus and a small (10-50ohms) 
series resistance will damp any ringing that results. 

However, once your off the bus (core card logic) you can do as you 
please as the system doesn't see it.  Use care on decodes to avoid 
spikes, timing races and other logic faults that can occur when 
using mixed speed logic. Generally good care will avoid this.  

The last area.  Power and gound busses. Since there is a lower 
likelyhood of having the DEC protocards any board constructed
should use good power distribution rules (better than PdP-8 days
as newer is faster!). 

The rest of the details are in the DEC logic handbooks and PDP-8 
handbooks. They are your friend, or try to be. ;)

Allison