[GreenKeys] Paul, ad7i, email about local loops

[Jim Haynes]

On Mon, 25 May 2020, Jeffrey Golas wrote:
> Its a logic module to an IBM mainframe, each one of these was one latch 
> or one counter per se. I have a similiar unit on my tv stand, and they 
> are worth a few bucks now.
>
I bought a few of those in the early 1960s and took them apart for the
parts.  They belonged to the IBM 700 series equipment, probably the
early 701 computer.  I traced out some of the circuitry.  Some of them
were a bit slice of the accumulator register, using the Havens delay
circuit instead of a flipflop to store a bit.  Don't know if I still
have one.

IBM was most famous for its sales organization, but I'm also impressed
by the manufacturing engineering.  The wire-contact relays, those 
pluggable units for the mainframe, and later threading core memories
by machine.

I did some work on a Burroughs 220 vacuum tube computer, and later took
care of a B-5500 transistor machine.  I'm not all that fond of the tubes
and germanium diodes.  The B-220 had a bunch of pluggable units, same
idea as the IBM machine but much less compact and more costly to produce.

Jim W6JVE