deskthority

Posted: **08 Dec 2015, 20:50**

I'm cross-posting this here from geekhack as an odd discovery. It turns out one of the keyboards I sold has an interesting hackjob on the controller, as shown in this thread.

I don't really know the history on these two, except a lot of boards came from Litton Data Systems, Boeing and Hughes - all of which are US defense contractors.

ITC P/N 700022-000. Nothing shows up for that, nor do I find anything for XKB-3. The phone number there seems to go to modern-day Honeywell so it may well be impossible to find information on the point of these.

This "Revision D1" lacks the foil on top of the Intel chip. Here's a picture of a stock controller: http://i.imgur.com/sfZd8E0.jpg

Nice big grounding bar to the bottom plate.

Additional ground strap added on. Looks like they stuck a longer bolt in the hole in order to get room for another nut.

Posted: **08 Dec 2015, 20:58**

Nice bodge on that controller!
It seems they are mostly bypass capacitors, maybe it was operating in a noisy (electrically) environment and had to fiddle with bypass to clean up the signals.

Posted: **08 Dec 2015, 21:29**

Here is a close up of the Rev E board.

: DSC00512-1920-crop.jpg (469.14 KiB) Viewed 1494 times

A buddy who has been in the defense business since the early 80's suggested that ITC is probably what is now ITC Defense Corp.

Posted: **08 Dec 2015, 22:38**

At first I thought I agreed they were RLC filters to deal with a noisy environment, but notice the last strobe line is heavily filtered - it's the one that is on the left-hand side of the keyboard, and fairly exposed - but it is a strobe, so it's pretty much noise-immune.
The way the IBM F works, it is the height of the voltage reflected through the cap that is measured, not the height of the spike at the front caused by the derivative of the voltage as is used to sense in the xwhatsit controller, or the topre sense - both of which rely on a steep strobe pulse.

Code: Select all

topre, xwhatsit sense the height of the spike at the '+' . (this is a rather brutal simplification of what topre does - apologies)
  STROBE                SENSE
                          +
     _____                A
     |    |     ----->    | \___
_____|    |_____     _____|     \_

IBM  senses on the flatter area after any initial spike - 
                          
     _____                A   +
     |    |     ----->    | \___
_____|    |_____     _____|     \_

which will also accept no significant initial spike:

       _____                        +
      /     \                      _____
     /       \         ------>    /     \
____/         \_____        _____/       \_____

So - I think that particular messy filter rounds the front and back of the strobe, so it will *cause* less interference with nearby hardware. I don't think it's a device to prevent people from snooping, as the result line back from the sense comparator - a relatively loud digital line - is not filtered.

dfj's $0.05

deskthority

ITC XKB-3 (Modified XT Keyboard)

ITC XKB-3 (Modified XT Keyboard)