> On Mar 29, 2025, at 4:06 PM, Milo Velimirović via cctalk
> <cctalk@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
