29 Mar
2025
29 Mar
'25
8:17 p.m.
On Mar 29, 2025, at 4:06 PM, Milo Velimirović via
cctalk <cctalk(a)classiccmp.org> wrote:
...
The ability to use larger printed circuit boards; it gets you higher density and avoids
the need to run (as many) interconnects off the board and thru the backplane. Remember
that 50-60 years ago was the era of 14,16-pin DIP packages for small-scale and
medium-scale
integration; larger packages existed of course, but they were the exception. You still
needed lots of wires to interconnect functionality between chip packages. Using a hex
board instead of single or dual boards allowed many of those “wires” to be traces on a
PCB, rather than run through the backplane or on an over-the-top jumper. (See the 11/34a.)
Also PCB technology. The early boards had lots of space between components. I believe
they were typically just two layers, with trace widths such that you could not run a trace
between DIP pins.
Once you get traces between pins you can get a lot more density; 4 or more board layers is
yet another major increment in possible density. Some of that isn't workable without
sufficiently fancy CAD tools. I don't know about DEC in the PDP-11 era, but I
remember doing my own PCB layout (for a university project): two layers, done with red and
green translucent adhesive tape on a light table, in 2x scale. To make matters even
stranger, the university board shop could do two layers but not plated through holes.
Occasionally you'd see single layer boards in DEC products. I remember them in the
VT61, a crazy looking board with hundreds of jumper wires on it. Why they didn't do
it as a two-layer board I have no idea, I would assume it would have been cheaper as well
as more reliable.
paul