CiTOH terminals (was Re: Well, now I've gone and done it... Dragged home more big(ish) iron...)
22 May
2007
22 May
'07
12:58 p.m.
On 5/22/07, Mr Ian Primus <ian_primus at yahoo.com> wrote:
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
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. 
22 May
22 May
10:22 p.m.
New subject: CiTOH terminals (was Re: Well,
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
I'm not suprised you had trouble understanding it. Bits of it are not at
all obvious.
I'm working for my (paper) VT100 printset, I hope it's similar enough to
the one on bitsavers (or wherever) to be able to follow me.
Anyway, you're right that it uses 6402 or similar UARTs. What makes the
interfave complciated is that there are only 3 wires (+12V power,
ground, and a single signal line) and that there's no clock oscillator
circuit in the keyboard. In other words, one line carries the clock from
the terminal to the keybard and data both ways.
Start on the 'VT1100 Basic Video' sheet 6 (BV6). The keyboard
communciation UART is E55. The transmit and receive clocks (same signal
on both) comes from LBA4 H, which is, I think, just a convenient clock
frequency (It stands for Line Buffer Address bit 4 I think).
The circuitry round E28 seems to be a pulse width modulator, controled by
the trasnmit data pin (Serial Out) of the UART. It sends a pulse to the
keybard, the width of which depends on the current bit to send.
The (analogue-ish) circuitry round E40b separates the data bits from the
keuyboard from the signal produced by said modulator, and sends the
incoming data to the UART.
OK, now to the keyboard. 'VT100 Full Keyboard' sheet 1 (KB1)
Again we kave a circuit to separate the incoming and outgoing data here
E2b. The incoming stream provides the clock to the UART, it also goes
into E2a, which would seem to be a monostable cirucit, which thus
demodulates the PWM signal. The output of that goes to the serial input
on the UART. Data from the UART foes via E9f and E5d and then back to the
terminal
E12 is, of course, the UART. Received data (8 bits) controls the LEDs,
speaker, and starts the scan (of which more in a momnet)
E7 and E10 are the scan counter, clocked, essentially by the same clock
that clocks the UART (but it can be paused while the UART is transmitting
by E4) At the end of a scan, E3b is set, thus stopping the counter by
holding it reset until the scan is restarted by the received data bit
mentioned above. When the scan gets to the position of a pressed key, KB2
KEY DOWN L is asserted, thsi causes the UART to transmit the current
value of the scan counter, so the key 'code' is transmitted back to the
terminal.
OK, over the page to sheet 2 (KB2). There's a nice, conventional matrix
of swtiches. The rows are driven by E11 and E13, which are controlled by
the bottom 4 bits of the scan counter. The cloums are selected one at a
time by E14, controleld by the top 3 bits of the scan counter. Thus each
switch is seelcted in turn and its state fef to KB2 KEY DOWN L. As I said
above, if a key is pressed, the scna count is sent back to the terminal.
Hope that helps a bit
-tony
23 May
23 May
1:02 a.m.
New subject: CiTOH terminals (was Re: Well,
On 5/22/07, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
Indeed it isn't, but I had a go at the VT100 Technical Reference Guide
I found a reference to via vt100.net, and more of it makes sense now,
especially the combination clock and bidirectional data line. That's
a lot of work for one wire. The 4-wire LK201 interface is much more
up my alley.
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
I'm not suprised you had trouble understanding it. Bits of it are not at
all obvious. Hope that helps a bit
Yes. Thanks, Tony!
-ethan
10:11 p.m.
New subject: CiTOH terminals (was Re: Well,
Indeed it isn't, but I had a go at the VT100
Technical Reference Guide
I found a reference to via vt100.net, and more of it makes sense now,
Argh! I didn't realise there was a technical manual (in any case I
couldn't easily read it...).
I just grabbed the printest, fliped over the pages a few times and it
became reasonably clear to me. Now quite what that says about me is
another matter...
especially the combination clock and bidirectional
data line. That's
a lot of work for one wire. The 4-wire LK201 interface is much more
up my alley.
Indeed. That one is a fairly normal asynchronous serial interface at a
standard baud rate (I forget what).
I wonder why the VT100 interface was so odd. Maybe to get it onto a 3 pin
connecotr so they could use the 3-pole jack plugs (as we call them over
here). But there are 4, 5 and 6 pole versions that were used on UK (GPO)
telepohones years ago. Maybe those never crossed the Pond...
-tony
24 May
24 May
noon
New subject: CiTOH terminals (was Re: Well,
On Wed, 2007-05-23 at 23:11 +0100, Tony Duell wrote:
I wonder why the VT100 interface was so odd. Maybe to
get it onto a 3 pin
connecotr so they could use the 3-pole jack plugs (as we call them over
Engineer 1: "I bet I can do it with power, ground and one signal wire"
Engineer 2: "I bet you can't"
Gordon
8:53 p.m.
New subject: CiTOH terminals (was Re: Well,
On Wed, 2007-05-23 at 23:11 +0100, Tony Duell wrote:
I'm suprised sobody at that point didn't suggest modulating the power
line and getting it down to 2 wires ;-)
I was once told a probably apocryphal story that the 11/730 came about
because an engineer bet he could fit a VAX CPU onto 3 hex-height cards
using only standard chips :-)
More seriosuly, I was once sort-of involved in a piece of digital design,
and there was some combinatorial block that took in 3 bus lines and
outputted an enable signal to a buffer chip. Without even thinking about
the problem, I said 'I can do that in one chip'. One of the others said
'OK, a PROM or a PAL I suppose'. I said 'No, A normal, non-programmed
chip from the TTL data bool'.
OK, which chip was I thinking off?
-tony
I wonder why the VT100 interface was so odd.
Maybe to get it onto a 3 pin
connecotr so they could use the 3-pole jack plugs (as we call them over
Engineer 1: "I bet I can do it with power, ground and one signal wire"
Engineer 2: "I bet you can't" 
9:58 a.m.
New subject: CiTOH terminals (was Re: Well,
Tony Duell wrote:
Guessing a active low enable. Hmm TTL ... 74150
:)
Almost any decoder comes to mind or some 3 input nands.
On Wed,
2007-05-23 at 23:11 +0100, Tony Duell wrote:
I'm suprised sobody at that point didn't suggest modulating the power
line and getting it down to 2 wires ;-)
I was once told a probably apocryphal story that the 11/730 came about
because an engineer bet he could fit a VAX CPU onto 3 hex-height cards
using only standard chips :-)
More seriosuly, I was once sort-of involved in a piece of digital design,
and there was some combinatorial block that took in 3 bus lines and
outputted an enable signal to a buffer chip. Without even thinking about
the problem, I said 'I can do that in one chip'. One of the others said
'OK, a PROM or a PAL I suppose'. I said 'No, A normal, non-programmed
chip from the TTL data bool'.
OK, which chip was I thinking off? I wonder why the VT100 interface was so odd.
Maybe to get it onto a 3 pin
connecotr so they could use the 3-pole jack plugs (as we call them over
Engineer
1: "I bet I can do it with power, ground and one signal wire"
Engineer 2: "I bet you can't" -tony
.
10:08 p.m.
New subject: CiTOH terminals (was Re: Well,
More seriosuly, I was once sort-of involved in a piece
of digital
design, and there was some combinatorial block that took in 3 bus
lines and outputted an enable signal to a buffer chip. Without even
thinking about the problem, I said 'I can do that in one chip'.
OK, which chip was I thinking off?
Depends. What levels did those three bus lines have to be to generate
the enable? If they're all high or all low, a three- (or more-)input
AND/NOR or NAND/OR (depending on whether the active level of the enable
is high or low).
If not, you could do it with an 8-to-1 mux with the inputs tied to
fixed levels, but it seems rather wasteful - this does, however, have
the advantage that you can run the inputs out to 8 jumpers and let the
user jumper the thing to appear at any desired subset of the 8 possible
places...or maybe that's a disadvantage; it depends.
For most combinations, you could do it with a quad NAND or NOR, but I'd
have to look at it exhaustively to be sure that's true for all eight
possibilities.
Also relevant is whether glitches are tolerable: is it acceptable for
the enable line to go active for a nanosecond or two in the presence of
certain transitions on the inputs that never actually include the magic
combination? If not, you have to make sure all the delays work out
properly.
I don't recall the numbers for any of the above chips even for the 74xx
family (which is the only one I know well enough to know those for).
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10:23 p.m.
New subject: CiTOH terminals (was Re: Well,
On Thursday 24 May 2007 16:53, Tony Duell wrote:
More seriosuly, I was once sort-of involved in a piece
of digital
design, and there was some combinatorial block that took in 3 bus
lines and outputted an enable signal to a buffer chip. Without even
thinking about the problem, I said 'I can do that in one chip'. One of
the others said 'OK, a PROM or a PAL I suppose'. I said 'No, A normal,
non-programmed chip from the TTL data bool'.
OK, which chip was I thinking off?
An 74138 3-to-8 demux? Or, possibly a 2-of-4 demux (74139), if you had
at least one line that had to be low.
Pat
--
Purdue University ITAP/RCAC --- http://www.rcac.purdue.edu/
The Computer Refuge --- http://computer-refuge.org
10:27 p.m.
New subject: CiTOH terminals (was Re: Well,
Tony Duell wrote:
More seriosuly, I was once sort-of involved in a piece
of digital design,
and there was some combinatorial block that took in 3 bus lines and
outputted an enable signal to a buffer chip. Without even thinking about
the problem, I said 'I can do that in one chip'. One of the others said
'OK, a PROM or a PAL I suppose'. I said 'No, A normal, non-programmed
chip from the TTL data bool'.
OK, which chip was I thinking off?
If all you want to do is select a single combination of three
bits and enable based on that, a 74138 will do it. If you want
a more complex set of enable conditions, a multiplexer like the
74151 would be better. Connect the three bits to the three
'select' inputs, and wire up the eight 'data' inputs to
generate the truth table you need. It's a bit like an 8-bit
ROM, in a way.
You can use it with four input bits, too, but you might need
an inverter as well.
--
John Honniball
coredump at gifford.co.uk
25 May
25 May
12:59 a.m.
New subject: CiTOH terminals (was Re: Well,
Tony Duell wrote:
Exactly. Without knowing anything about the actual function, I knew I
could do it with a 74x151 (acutally, a 74F151) 8 input mux. The fact that
I wsa using the output to enable a data buffer rather than, say, to clock
a counter, means that odd glitches on that output of said circuit didn't
matter.
More seriosuly, I was once sort-of involved in a
piece of digital design,
and there was some combinatorial block that took in 3 bus lines and
outputted an enable signal to a buffer chip. Without even thinking about
the problem, I said 'I can do that in one chip'. One of the others said
'OK, a PROM or a PAL I suppose'. I said 'No, A normal, non-programmed
chip from the TTL data bool'.
OK, which chip was I thinking off?
If all you want to do is select a single combination of three
bits and enable based on that, a 74138 will do it. If you want
a more complex set of enable conditions, a multiplexer like the
74151 would be better. Connect the three bits to the three 'select' inputs, and wire up the eight
'data' inputs to
generate the truth table you need. It's a bit like an 8-bit
ROM, in a way.
It is _exactly_ like an 8 bit ROM, in that it consists of a fixed AND
matrix (the decoder section of the mux) follwed by a prograamble OR
matrix (here just one output).
You can use it with four input bits, too, but you might need
an inverter as well.
Yep. Or alternatively for the 3 inputs, I could have used the 74x153
4-input mux and maybe a single inverter.
-tony
6440
days inactive
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10 comments
7 participants
participants (7)
-
ard＠p850ug1.demon.co.uk -
bfranchuk＠jetnet.ab.ca -
coredump＠gifford.co.uk -
ethan.dicks＠gmail.com -
gordon＠gjcp.net -
mouse＠Rodents.Montreal.QC.CA -
pat＠computer-refuge.org