Honeywell Micro Switch and USB interface

aestechnic

23 Jun 2020, 18:32

Hi all,

I wanted to report on a Micro Switch SD board which I'm building a USB interface for. It is an SD-16279 pcb, and the case has the label "ti BKBD002B-002". I think these keyboards were originally used with a Honeywell VIP 7200 or 7205 terminal; here is the manual:

https://archive.org/details/bitsavers_h ... 81_2667150

I found several similar keyboards on this forum. They are never quite identical, but I think lucar's in the following list has the same PCB:

snuci: viewtopic.php?f=62&t=15943
Firebolt1914: viewtopic.php?f=2&t=9970
lucar: viewtopic.php?f=2&t=18542

Most switches on my board are 4A3S (scan type, in contrast to snuci's above which uses pulse type 4A3B). The exceptions are the Caps lock, which is 5A3S (it physically stays down until you press it again to release it) and the bigger keys with a passive support "switch", these use 4A1S instead, which has less spring resistance.

Here is the board, there are many more better photos of the similar boards in the threads linked to above (the cable is hardly original, I think the wiring matches a bidirectional IBM PC parallel port):
Spoiler:
outside.jpg
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inside.jpg
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I was happy to find that the PCB is single sided, so it is easy to read, and I made a diagram of the whole thing (there may be errors, of course, and also it is a bit messy as I just drew it as I went along "reading" the PCB and I haven't cleaned it up afterwards). Here it is, minus the switch matrix:
Spoiler:
sd16279.png
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Quick summary of the schematics: the controller is an 8048 and it buffers its output with a pair of 74368 hex inverting buffers (three of those gates are also used as an oscillator). Then a 74154 demultiplexer is used to scan the switches (I used the labels "IN" and "OUT" from the viewpoint of the switches, so the demultiplexer outputs going to the keyboard matrix are labelled "SW_IN"). The switch outputs SW_OUT are connected directly to the "DB" port of the 8048 (with 3 exceptions, see below). I'm guessing the funny resistors R8-R11 are spare inputs (require desoldering of some wire jumpers).

First I wanted to see what I can get from the existing interface through the keyboard's 25-pins connector (it's 26 pins in my diagram, but the 26th pin doesn't exist physically). It is a parallell port with strobe. The strobe (pin 10) goes low when a key is pressed, and then an 8 bit code can be read (pins 19-25 plus 13). The code is the ASCII code for keys were this makes sense. Key release is not signalled in any way. SHIFT/CTL keys cannot be read directly, but SHIFT+key and CTL+key (and CAPS+key) produce codes pretty much as one would expect.

In addition, there are 4 special output lines (pins 8,9,11,12 on the connector). Each goes low when RPT, END-OF-MESSAGE, BREAK or the BLANK key is pressed. They stay low for the duration of the key press and go high only when the key is released. So the state of these four keys is fully available and independent of everything else.

In fact, the keys RPT, END-OF-MESSAGE and BREAK are not connected to the 8048 at all, but have their inputs hardwired to ground (=active) and the output directly wired to the connector. (In particular, the RPT key is not handled by the controller, it does not cause keys to repeat.) Although the BLANK key in practice behaves the same way, it is different in hardware: it is scanned by the 8048 just like all the other keys, and it is the 8048 that drives the corresponding line in the connector.

One could make a reasonable USB interface connecting only to the output port as it is, perhaps using the special keys RPT, END-OF-MESSAGE, BREAK and BLANK as modifiers. Somewhat better control of the keyboard's state is of course desirable, but I also wanted to keep the board as untouched as possible. I ended up to piggy back on the controller: I connected an Arduino Uno (for testing, eventually to be replaced by a Tweeny) and let the original controller do its thing with me spying:
Spoiler:
arduino.jpg
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This works. I listen to the four SW_IN lines (pins 21-24 on the 8048) and wait for the transition from 0 to 15=1111b (the 8048 scans downwards). Then I record the SW_OUT lines (pins 27--34 on the 8048), wait for SW_IN=14, record the SW_OUT lines etc. I also timed the 8048: it holds SW_IN fixed for roughly 150µs, except for the value 15=1111b, which is held only for around 70µs. This is possibly because only the SHIFT and CTL keys are connected to the SW_IN=15 line, so I can imagine that the 8048 code handling this line is different from the rest (incidentally, the SW_IN=15 line together with SW_OUT4, SW_OUT5, SW_OUT7 are also exposed through a 4-pole edge connector labelled "TP", I guess for extending the keyboard with up to 3 external switches). I also need to monitor the END-OF-MESSAGE, RPT and BREAK keys through the 25-pins connector. In this way I can read the entire keyboard state without interfering with the keyboard's original function, and it could still be connected to a VIP terminal. I think I am done at this point as it now is straight forward to make a USB interface; I just have to wait for my Teensy to arrive in the mail.

Unfortunately, two of the keys turned out to be dead. One 4A3S (the "6&") and one 4A1S (the ERASE). Bummer. I'll desolder them to confirm that they are indeed dead, but as they sit in the middle of the kbd matrix and all other keys work, I am not optimistic. I'll probably have to swap them with two keys I can live without.

Just hoping writing this up would be useful to someone. Do ask for clarification or further details (and contribute corrections; I wrote this somewhat quickly).

[Edit: The "In addition..." paragraph had somehow disappeared.]
Last edited by aestechnic on 24 Jun 2020, 19:07, edited 2 times in total.

Slom

23 Jun 2020, 21:19

That was a nice read :) Thank you.

I also might need to get one of those IC-Clips, that's a cool idea to avoid modifying PCBs.

aestechnic

24 Jun 2020, 08:51

Slom wrote:
23 Jun 2020, 21:19
That was a nice read :) Thank you.

I also might need to get one of those IC-Clips, that's a cool idea to avoid modifying PCBs.
I'm rather proud of teenage me, getting hold of that clip and understanding that at some point during the next 25 years or so I would probably need it!

But I will eventually solder a Teensy to the board, I'm not that minimally-invasive. The main point was to still have the original controller running.

User avatar
inmbolmie

24 Jun 2020, 13:07

I like this concept of "piggybacking" the existent controller, it is clever and allows to keep the keyboard as original as possible.

How do you plan to power the original controller? From the USB? Does it require any weird voltages?

aestechnic

24 Jun 2020, 15:32

inmbolmie wrote:
24 Jun 2020, 13:07
I like this concept of "piggybacking" the existent controller, it is clever and allows to keep the keyboard as original as possible.

How do you plan to power the original controller? From the USB? Does it require any weird voltages?
Thanks, I was also happy with that solution.

Power: yes, USB, it's just plain 5V logic. I didn't measure power consumption but it works fine with USB power via the Arduino in the picture.

User avatar
lucar

24 Jun 2020, 17:44

Hello aestechnic,

You mentioned me , here I am!

Here are some pictures of my white Honeywell, for comparison. I think we have the same unit.

Image

Image

Image

Image

Image

I also have the original cable..

Image

You made a great work rebuilding the schematic, how can I help?

Luca

aestechnic

26 Jun 2020, 09:31

Luca: Hi! Yes, same board, it seems, the "CUST. PART" number matches as well. But the exterior "ti" label is different, yours is BKBD002A-003 and mine is BKBD002B-002. Whatever that means. That 3M-connector, what is it? It isn't DSUB?

User avatar
lucar

26 Jun 2020, 15:07

It's DSUB , it's not clear from the picture. It's the same cable You have, only a different brand. The interesting fact is that we have the part number!

aestechnic

26 Jun 2020, 17:21

lucar wrote:
26 Jun 2020, 15:07
It's DSUB , it's not clear from the picture. It's the same cable You have, only a different brand. The interesting fact is that we have the part number!
Mine looks a bit home made (although well made), it uses only a single wire each for GND and 5V and it does not connect through the lines corresponding to the special keys END-OF-MESSAGE, RPT, BRK and BLANK.

aestechnic

02 Sep 2020, 09:25

Here's an update with the finished project.

Teensy 2.0 spying on the 8048 controller:
teensy.jpg
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I desoldered the LED on D6. The Teensy has enough inputs anyway, but the physical location of D6 was practical. The board is powered via USB, I think it is about 4-500 mA. Strictly speaking the Teensy doesn't tell the host it needs more than 100 mA (the USB enumeration business), but for a hobby project I'm happy.

And here is were it picks up the 3 special keys that are "outside the matrix":
wires.jpg
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Code (boot protocol, very basic) on github (hope I got that right, still haven't really converted from svn):
https://github.com/aestechnic/piggyswitch

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