CiTOH terminals (was Re: Well, now I've gone and done it... Dragged home more big(ish) iron...)
22 May
2007
22 May
'07
11:02 a.m.
Hi
I have just remembered how the later version keyboards worked.
It was still a counter sytem but the values where fully decoded.
It worked on a matrix system.
Each key on the keyboard sat across the junction of one of the matrix
points.
The columns, say 16. Would be the output of a 4 in 16 out decoder and
the rows were the
inputs of a say 16 in four out encoder. Stopping the clock by pressing a
key gave you an 8 bit keycode.
Serialize in the usual way.
Finally dedicated chips appeared. But same old matrix system.
Rod Smallwood
-----Original Message-----
From: cctech-bounces at classiccmp.org
[mailto:cctech-bounces at classiccmp.org] On Behalf Of Ethan Dicks
Sent: 22 May 2007 13:58
To: General Discussion: On-Topic and Off-Topic Posts
Subject: CiTOH terminals (was Re: Well,now I've gone and done it...
Dragged home more big(ish) iron...)
On 5/22/07, Mr Ian Primus <ian_primus at yahoo.com> wrote:
The 101 is the model shaped and colored much like
a VT100. The
keyboard looks similar... but its protocol is _not_ compatible with
the VT100.
Good thing I didn't buy the one on eBay a couple months ago. There was a 101 there, and cheap too, but it had no keyboard.
I may haev a few CiTOH terminals to drag to VCFmw if there's any
interest. Closer to the event, I'll inventory what I have and see if I
kept any spare keyboards "just in case", if anyone out there already has
a terminal, but can't figure out why their VT100 keyboard doesn't work
with it.
I remember back in the day, since we had dozens of CiTOH and DEC
terminals, looking over the available schematics for both and not being
able to figure out how the keyboard works. I think it was a matter of
inadequate/fuzzy docs more than anything else. Does anyone know of a
good printset to pore over to see the nuts-and-bolts of a VT100-era
keyboard? ISTR the crux of it was a 6402-type UART squeezing out the
keystrokes at some slow baud rate, but I can't recall any essential
details right now. I'm just curious if it's possible to swap a crystal
or make a simple, switched change to allow one keyboard to work across
both vendor's product lines.
We used to have lots of dead keyboard when there was one or two
terminals on everyone's desk. Since the company was shrinking at that
stage, we never bothered fixing them - we just pulled one off a vacant
desk and kept working. The number of working keyboards never shrank
below the steadily decreasing size of the staff, so economically, it
made sense. I think I only saved working keyboards in that set of 4 van
loads, but it's entirely possible I picked up one or two dead ones.
Right now, I have to search the pile for a VT100 keyboard to get my
DECmate I back up and running so I can press it into service as an
RX01/RX02 image archiver and finally whittle down my cartons of
floppies.
-ethan
22 May
22 May
2:51 p.m.
On 5/22/07, Rod Smallwood <RodSmallwood at mail.ediconsulting.co.uk> wrote:
Hi
I have just remembered how the later version keyboards worked.
It was still a counter sytem but the values where fully decoded.
It worked on a matrix system.
Each key on the keyboard sat across the junction of one of the matrix
points.
Thanks for those technical tidbits, Rod. What you wrote resonates
with what I remember of my explorations of 15 years ago. The UARTs in
the keyboards were of the type that worked like a PISO shift register,
but with start and stop bits "for free"; no CPU required to set up the
UART - all pin-selectable options (like the INS6402, as I said). I'll
have to cast a careful eye at how the matrix ties into the UART and I
think this will all make more sense.
Since I first tried to tackle this, I've spent a lot of time reading
the "CMOS Cookbook" and the "TTL Cookbook" - lots of strange
counter-based things in there. I was just learning how logic devices
functioned back in their heyday, so I'm OK with straightforward uses,
but some of the really clever tricks I have to study before I get.
Thanks again, Rod.
-ethan
5:40 p.m.
New subject: CiTOH terminals (was Re: Well,
Hi
I have just remembered how the later version keyboards worked.
It was still a counter sytem but the values where fully decoded.
It worked on a matrix system.
Each key on the keyboard sat across the junction of one of the matrix
points.
One of the oddest ones I've worked on was the HP98203 -- the little
keyboard used on the HP9816.
The interface has 2 signal lines, power and ground. One signal is a clock
line from the machine to the keyboard. The other is a bidirectional line,
from the machine to the keyboard it indicates the start
of a scan, from
the keyboard to the machine it indicates the state of a
'key'.
After the reset, the thing effectively sends 128 bits, clocked by the
clock line. These represent not only the states of all the keys, but also
the state of 8 internal configuration links, and 8 bits for the
twiddleknob counter and direction.
Now for the odd part. The keyboard swtiches are not wired as a matrix.
One side of each switch is grounded. The other side goes to an input on a
4051 mux. Scanning is done by a 7 bit counter (4024 IIRC) and a 1-16
decoder (4514) to select the individual 4051s.
No idea why they did it this way, but they did.
The other thing to watch for are keybaords that don't use contact closure
as a way of detecting a key is pressed. There have been inductive
keyboards (pressing a key completes a magnetic core, thus increading the
coupling between a couple of tracks/wires (e.g. the Termiprinter).
Balanced transformer keybaord, whenre presisgn a key damps one of a pair
of pulse transformers allowing a signal from the other one to get through
(e,g. HP9810). Saturated core keyboards, where 2 wires are linked by a
magnetic core, which is normally satureated by a permanent magnet.
Pressing a key moves the magnet away, increasding the coupling between
the wires (e,.g. Nascom, HP9845). Capactitive keyvoards (Keytronics) are
common, but HP used a capacitive membrane mechanism in the HP46021 IIRC.
-tony
6267
days inactive
6267
days old
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ard＠p850ug1.demon.co.uk -
ethan.dicks＠gmail.com -
RodSmallwood＠mail.ediconsulting.co.uk