S-100 backplane board spacing

[Allison]

>
>Subject: S-100 backplane board spacing
>   From: "Andrew Lynch" <lynchaj at yahoo.com>
>   Date: Thu, 05 Feb 2009 22:32:10 -0500
>     To: <cctalk at classiccmp.org>
>
>Hi!  Is there any sort of a standard for S-100 backplane spacing?  
>
>I measured a couple of backplanes I have (a VG and another one) and they
>both measured .375" (3/8") gap between the connectors.  That seems rather
>closely spaced to me if the boards are 3/4" apart if the connectors are 3/8"
>wide themselves.
>
>What would be the minimum useful gap between S-100 connectors for a small
>hobbyist S-100 backplane?  I presume 3/8" gap is not enough if you want to
>accept prototype boards with wire wrap sockets and maintain some clearance.

There are two considerations, airflow and bus length.  Wider gap helps 
airflow to a point then wider doesnt help as much.  Wider spacing makes
the bus longer for a give number of connectors and longer has more problems
with reflections(ringing) and other transmission line behavours.  Longer may
also impact power distribution as well.  Last items is the height of boards
limits how close or forces skipping slots.  Some fo the taller boards in my 
collection suggest greater than 0.75" center to center would barely be 
adaquate because of tall heatsinks contacking the next board.

Look at the better backplanes, Compupro would be one.


Also the later boards (more modern) were dense enough that only a few 
slots are required.

I have at least two machines that are:

 Zpu-B z80 cpu
 64K ram (ram 16, 17 or 22)
 Interfacer (multi serial plus parallel)
 DISK1A  FDC
 DISK3   HDC
 Systemm Support 

Thats only 6 boards and the smallest backplane I have is 8 slots. It's as loaded
a system as most or more so.  The limiting factor for the box is fans, power supply
and mountings for the disks.

Older antiques like an altair filled with then current board may have:

  CPU  8080or later Z80
  Ram boards anhywhere from 4k per to 16K per (as many as 8 for 64K)
  Some form of rom boot card
  possible a dual board FDC (altair disk was a two card set)
  multiple IO baord for serial
  possible multiple boards for one or more printers or other parallel devices.

  And cooling issues.


Hope that helps with some history.  I'd say 6-8 slots spaced 1" and also on 
the board terminator.


Allison

>
>Any constructive thoughts on the subject much appreciated.
>
>Thanks and have a nice day!
>
>Andrew Lynch