16 Feb
2003
16 Feb
'03
4:56 p.m.
Well, my DEC LA120 is almost perfectly operational. Its only problem is a
set of keys that do not work when pressed. I'd like to fix that.
What I see when I remove the keycap is a square plastic housing that slides
vertically within a larger square plastic housing. The smaller housing
slides down when one presses a key. A spring below the smaller square
housing pushes the housing back up when the key is no longer pressed. Up
through the smaller housing shoot two electrical contacts. The contacts are
fixed and do not slide with the housing. When the key is up, a plastic bar
across the middle of the smaller housing holds the two contacts apart. When
the key moves down, the bar moves down and no longer holds the contacts
apart. The contacts touch and complete a circuit, and the LA120 senses a
keystroke.
Top view (key not pressed):
+-A------------------+
| +-B-----+-+------+ |
| | | | | |
| | | | | | | |
| | D |C| E | |
| | | | | | | |
| | | | | |
| +-------+-+------+ |
+--------------------+
Side view (key pressed):
| | /\ | |
A B / \ | |
| | D E | |
| | / +--+ \ | |
| | / |C | \ | |
| | / +--+ \ | |
| +----------------+ |
| /\/\/\/\F/\/\/\/\/ |
+--------------------+
A = Outer (fixed) housing
B = Inner (sliding) housing
C = Bar that holds contacts apart when not pressed
D = Electrical contact
E = Electrical contact
F = Spring
The problem is that the circuit isn't being closed when the key is pressed.
If I stick a screwdriver in there, bridge the gap between the contact, and
thus close it manually, a keypress is sensed. Actually, it senses several
keypresses very quickly, probably because of the noise caused by the
conductive screwdriver scraping across the contacts. When I press the
housing down, the contacts *appear* to touch, but no keystroke is sensed. So
I figure there are either or both of two possible things going on:
(1) Tiny space between the contacts
(2) Nonconductive material (corrosion? oxidation?) on the contacts
Two keys were fixed by using the screwdriver to bend the contacts toward
each other in the hopes of creating more force pushing them towards each
other. The ENTER key does not seem to be responding well to that treatment.
One thing is for sure. All this stuff is so tiny and hard to get to that it
is difficult to work on.
Any suggestions?
--
Jeffrey Sharp
16 Feb
16 Feb
11:50 p.m.
Jeffrey,
the contacts you describe from your LA120 (cannot look at my own right
now) seem to be similar to what I saw in a VT100 keyboard. This keyboard
had been stored in a place were mice and rabbits lived, but it recovered
from initial contact problems just by continued use:
the isolating
plastic has a cleaning effect on the electrical contacts as it slides
between them.
In general, I often have success when cleaning contacts by wiping them
with paper: it's rough enough to remove corrosive layers, but soft
enough to keep the contacts intact, and it's readily available. Best way
to apply is: put paper between contact fingers while the switch is open,
then close switch, and, while applying little or no additional pressure
on the contacts, move paper back and forth a bit.
You will sometimes see a line left on the paper, especially if the
contacts are very dirty. This line proves that the method works.
Given the limited space around your keyboard contacts, you'll probably
want to cut a narrow stripe of paper, insert it with the help of a
screwdriver because you cannot open the contacts otherwise, then take
out the screwdriver and just pull out the paper from under the contact.
For this type of mechanical contact, a chemical solvent may also be
appropriate. I don't use contact cleaners on variable resistors (and
some other stuff) because it solves the problem only for a limited time,
and when the problem reappears afterwards, it's worse than before. A
"volume" potentiometer treated this way will crackle even louder in
about a month or two.
BTW: nice ASCII graphics! :-)
Andreas
Jeffrey Sharp wrote:
Well, my DEC LA120 is almost perfectly operational.
Its only problem is a
set of keys that do not work when pressed. I'd like to fix that.
...
Any suggestions?
--
Andreas Freiherr
Vishay Semiconductor GmbH, Heilbronn, Germany
http://www.vishay.com
17 Feb
17 Feb
midnight
I've had pretty good luck fixing keyboards like that. First, take the
keytop off. Then pull out the inner plastic housing. To do this, you'll
need to use some small pliers and be very careful not to damage it.
Once it is out, remove the spring. Now you have better access to the
contacts. Take a scrap of paper and get it wet with isopropyl alcohol,
then put it between the contacts while pressing them together a little
bit with a screwdriver or something, and pull the paper out. This does
a pretty good job of cleaning the contacts, and I have also had luck
with dry paper. Rougher paper helps, and if they are really dirty, I
would imagine you could try using some very fine grit sandpaper, being
careful not to damage the contacts. Once you've cleaned the contacts,
put the spring back in, and the inner plastic housing (be sure to get
the plastic bar going the right direction!). I have used this technique
to fix my DEC VT100 keyboard, as well as some other terminal keyboards
that have a similar mechanism. Good luck!
Ian Primus
ian_primus(a)yahoo.com
On Monday, February 17, 2003, at 01:54 AM, Jeffrey Sharp wrote:
Well, my DEC LA120 is almost perfectly operational.
Its only problem
is a
set of keys that do not work when pressed. I'd like to fix that.
What I see when I remove the keycap is a square plastic housing that
slides
vertically within a larger square plastic housing. The smaller housing
slides down when one presses a key. A spring below the smaller square
housing pushes the housing back up when the key is no longer pressed.
Up
through the smaller housing shoot two electrical contacts. The
contacts are
fixed and do not slide with the housing. When the key is up, a plastic
bar
across the middle of the smaller housing holds the two contacts apart.
When
the key moves down, the bar moves down and no longer holds the contacts
apart. The contacts touch and complete a circuit, and the LA120 senses
a
keystroke.
Top view (key not pressed):
+-A------------------+
| +-B-----+-+------+ |
| | | | | |
| | | | | | | |
| | D |C| E | |
| | | | | | | |
| | | | | |
| +-------+-+------+ |
+--------------------+
Side view (key pressed):
| | /\ | |
A B / \ | |
| | D E | |
| | / +--+ \ | |
| | / |C | \ | |
| | / +--+ \ | |
| +----------------+ |
| /\/\/\/\F/\/\/\/\/ |
+--------------------+
A = Outer (fixed) housing
B = Inner (sliding) housing
C = Bar that holds contacts apart when not pressed
D = Electrical contact
E = Electrical contact
F = Spring
The problem is that the circuit isn't being closed when the key is
pressed.
If I stick a screwdriver in there, bridge the gap between the contact,
and
thus close it manually, a keypress is sensed. Actually, it senses
several
keypresses very quickly, probably because of the noise caused by the
conductive screwdriver scraping across the contacts. When I press the
housing down, the contacts *appear* to touch, but no keystroke is
sensed. So
I figure there are either or both of two possible things going on:
(1) Tiny space between the contacts
(2) Nonconductive material (corrosion? oxidation?) on the contacts
Two keys were fixed by using the screwdriver to bend the contacts
toward
each other in the hopes of creating more force pushing them towards
each
other. The ENTER key does not seem to be responding well to that
treatment.
One thing is for sure. All this stuff is so tiny and hard to get to
that it
is difficult to work on.
Any suggestions?
--
Jeffrey Sharp
12:16 p.m.
Well, my DEC LA120 is almost perfectly operational.
Its only problem is a
set of keys that do not work when pressed. I'd like to fix that.
What I see when I remove the keycap is a square plastic housing that slides
vertically within a larger square plastic housing. The smaller housing
slides down when one presses a key. A spring below the smaller square
This was a very common design of keyboard at one time. DEC used it
(VT100, LA120, etc). So did HP (HP85, etc). And Radio Shack (early Model
1s).
housing pushes the housing back up when the key is no
longer pressed. Up
through the smaller housing shoot two electrical contacts. The contacts are
fixed and do not slide with the housing. When the key is up, a plastic bar
across the middle of the smaller housing holds the two contacts apart. When
If it's the standard design, one contact is sold and fairly stiff, the
other has the end cut into about 4 fingers, and is much more flexible.
the key moves down, the bar moves down and no longer
holds the contacts
apart. The contacts touch and complete a circuit, and the LA120 senses a
keystroke.
[...]
(1) Tiny space between the contacts
(2) Nonconductive material (corrosion? oxidation?) on the contacts
Two keys were fixed by using the screwdriver to bend the contacts toward
each other in the hopes of creating more force pushing them towards each
other. The ENTER key does not seem to be responding well to that treatment.
One thing is for sure. All this stuff is so tiny and hard to get to that it
is difficult to work on.
OK, remove the keycap (but you've done that). The plunger (inner housing
to you) will also pull out if you grip it with pliers. HP claim you
should always use a new one when reassmbling, but it's not really
necessary. When you've got that out, remove the spring too.
Then try cleaning the contacts in place. Take a piece of card (a bit of
Hollerith card is ideal if you have some). Soak it in propan-2-ol and
carefully work it between the contacts. After doing that check that the
contacts test as shorted on the track side of the PCB (work on one
switch at a time so as not to have sneak paths through other dismantled
switches -- a dismanteld switch is, of course 'closed').
If it still doesnt work, remove the contacts. Desolder them from the PCB
and pull out the stiffer contact with pliers, then _carefully_ remove the
other one. Clean them, bend the 'figered' contact slightly to increase
the pressure between them. Then put them back. There was a special tool
for this, but it's not essential.
-tony
12:59 p.m.
New subject: RS-232 Problem (Configuration ?)
Let me start off by saying "I hate RS-232". Anyway, now that that's
out of the way, I am having a problem with an RS-232 interface on an
IMSAI SIO board.
I've been debugging a North Star drive and I was having problems with
it booting (DOS 2) in my IMSAI. I would get the "*" prompt but OS
would not recognize my Soroc terminal input. At first I thought it
might be buggy code on my DOS floppy.
Anyway, to make a long story short, I have multiple of everything and
have ruled out everything but something to do with my RS-232
configuration (I guess). I have 2 SIO board and both behave the same
way.
I'm documenting what I have here, in case someone sees something
wrong that I've overlooked. BTW, this was previously working without
any different configuration or hardware, with the exception of the RS-
232 cable from the Soroc to the IMSAI. This is a brand new cable
that was working just a few weeks ago so I'm having a hard time
believing there is anything wrong with this cable. I have only one,
if you can believe that. I'm going to get another this weekend, if I
have not figured this problem out by then.
Anyway...
I'm using Port A (on the SIO) and have configured the A3 Jumper to
have
TD Pin 1 to Pin 15 RD
RD Pin 2 to Pin 16 TD
RTS Pin 4 to Pin 12 CTS
CTS Pin 5 to Pin 13 RTS
DTR Pin 7 to Pin 9 DSR
DSR Pin 8 to Pin 10 DTR
By the way, the SIO manual shows DTR-DSR as through straight throuh
in stead of crossover, as shown above. Seems that it should be
crossed over like the others. In any case, I have tried both ways
but it doesn't seem to make any difference.
I'm using a straight through DB-25 cable from my Soroc to my IMSAI.
I'm using the following CIN and COUT routines.
2900 DB03 CIN IN STAT
2902 E602 ANI IBIT
2904 CA0029 JZ CIN
2907 DB02 IN DATA
2909 E67F ANI 7FH
290B C9 RET
290C DB03 COUT IN STAT
290E E601 ANI OBIT
2910 CA0C29 JZ COUT
2913 78 MOV A,B
2914 D302 OUT DATA
2915 C9 RET
Areas I'm wondering about.
1) Is the A3 header correct for the IMSAI as the computer end and the
Soroc as the terminal. 3 pairs crossed over. It does write to the
Terminal properly and when I press the Break Key, I get an echo from
the IMSAI of *?, which seems normal.
I have tested the input routine, which is part of the DOS input
routines, and was working previously, by modifying it to display the
status to the programmed output on the IMSAI with this code.
0000 DB03 CIN IN STAT
0002 DBFF OUT FFH (programmed output port)
0004 E602 ANI IBIT
0006 CA0000 JZ CIN
0009 DB02 IN DATA
290B E67F ANI 7FH
290D C30000 JMP CIN
Seems that this should show whatever is read in the status port for
Port A. But when executed, the status never changes after I tap any
key.
My system is behaving as if it just can't receive any data from the
terminal. It's driving me buggy as this is a relatively simple
problem. Like I said, I have multiple SIO and they all behave the
same. I even tried another terminal and get the same results. I
have also even tried different ribbon cable from the SIO to the RS-
232 connector.
Anyone see something bonehead that I overlooked?
Thanks,
18 Feb
18 Feb
5:49 a.m.
New subject: RS-232 Problem (Configuration ?)
Let me start off by saying "I hate RS-232".
Anyway, now that that's
I know the feeling. I do battle with it far too often, and it takes too
much of my time. No 2 manufacturers seem to use the signals the same way...
out of the way, I am having a problem with an RS-232
interface on an
IMSAI SIO board.
I've been debugging a North Star drive and I was having problems with
it booting (DOS 2) in my IMSAI. I would get the "*" prompt but OS
OK, I have the schematic in front of me.
would not recognize my Soroc terminal input. At first
I thought it
might be buggy code on my DOS floppy.
Anyway, to make a long story short, I have multiple of everything and
have ruled out everything but something to do with my RS-232
configuration (I guess). I have 2 SIO board and both behave the same
way.
I'm documenting what I have here, in case someone sees something
wrong that I've overlooked. BTW, this was previously working without
any different configuration or hardware, with the exception of the RS-
232 cable from the Soroc to the IMSAI. This is a brand new cable
that was working just a few weeks ago so I'm having a hard time
believing there is anything wrong with this cable. I have only one,
if you can believe that. I'm going to get another this weekend, if I
have not figured this problem out by then.
Anyway...
I'm using Port A (on the SIO) and have configured the A3 Jumper to
have
TD Pin 1 to Pin 15 RD
RD Pin 2 to Pin 16 TD
RTS Pin 4 to Pin 12 CTS
CTS Pin 5 to Pin 13 RTS
DTR Pin 7 to Pin 9 DSR
DSR Pin 8 to Pin 10 DTR
OK, that seems like a possible configuration. The schematic shows that
pins 9, 10, 12, 13, 15 ,16 go to pins on the cable connector, so the
correct wiring of this jumper block depends on the cable.
[...]
1) Is the A3 header correct for the IMSAI as the
computer end and the
Soroc as the terminal. 3 pairs crossed over. It does write to the
Terminal properly and when I press the Break Key, I get an echo from
the IMSAI of *?, which seems normal.
The fact that you get valid output from the Imsai to the terminal
suggests that you've not got the connections crossed over (i.e the data
output at one end is going to the input at the other).
What have you done with pin 3 of the A3 jumper? Ift surely should be tied
low (disabling the current loop circuit), or the '08 gate at C4a will be
inhibited. This will prevent data input.
Anyone see something bonehead that I overlooked?
Waht I would do at this point is look at the RxD and handshake input pins
at the 8251 USART at B5 on the board.See if you can get a data stream at
the RxD pin, and that the handshake lines are asserted (low). If not,
then you know where to start looking.
-tony
6:03 a.m.
New subject: RS-232 Problem (Configuration ?)
The problem turned out to be a conflict between two different I/O boards.
I had an MIO in the system and apparently it was mapped to the same
location as the SIO. So when the control was read, it was an ANDING of
the status from both USARTS (I'm presuming here; have not actually checked
out the MIO yet). But when I removed the MIO board, it all began to work
properly.
So, problem is solved. Thanks,
Mike
Let me start
off by saying "I hate RS-232". Anyway, now that that's
I know the feeling. I do battle with it far too often, and it takes too
much of my time. No 2 manufacturers seem to use the signals the same
way...
out of the way, I am having a problem with an
RS-232 interface on an
IMSAI SIO board.
I've been debugging a North Star drive and I was having problems with
it booting (DOS 2) in my IMSAI. I would get the "*" prompt but OS
OK, I have the schematic in front of me.
would not recognize my Soroc terminal input. At
first I thought it
might be buggy code on my DOS floppy.
Anyway, to make a long story short, I have multiple of everything and
have ruled out everything but something to do with my RS-232
configuration (I guess). I have 2 SIO board and both behave the same
way.
I'm documenting what I have here, in case someone sees something
wrong that I've overlooked. BTW, this was previously working without
any different configuration or hardware, with the exception of the RS-
232 cable from the Soroc to the IMSAI. This is a brand new cable
that was working just a few weeks ago so I'm having a hard time
believing there is anything wrong with this cable. I have only one,
if you can believe that. I'm going to get another this weekend, if I
have not figured this problem out by then.
Anyway...
I'm using Port A (on the SIO) and have configured the A3 Jumper to
have
TD Pin 1 to Pin 15 RD
RD Pin 2 to Pin 16 TD
RTS Pin 4 to Pin 12 CTS
CTS Pin 5 to Pin 13 RTS
DTR Pin 7 to Pin 9 DSR
DSR Pin 8 to Pin 10 DTR
OK, that seems like a possible configuration. The schematic shows that
pins 9, 10, 12, 13, 15 ,16 go to pins on the cable connector, so the
correct wiring of this jumper block depends on the cable.
[...]
1) Is the A3 header correct for the IMSAI as the
computer end and the
Soroc as the terminal. 3 pairs crossed over. It does write to the
Terminal properly and when I press the Break Key, I get an echo from
the IMSAI of *?, which seems normal.
The fact that you get valid output from the Imsai to the terminal
suggests that you've not got the connections crossed over (i.e the data
output at one end is going to the input at the other).
What have you done with pin 3 of the A3 jumper? Ift surely should be
tied low (disabling the current loop circuit), or the '08 gate at C4a
will be inhibited. This will prevent data input.
Anyone see something bonehead that I overlooked?
Waht I would do at this point is look at the RxD and handshake input
pins at the 8251 USART at B5 on the board.See if you can get a data
stream at the RxD pin, and that the handshake lines are asserted (low).
If not, then you know where to start looking.
-tony
7980
days inactive
7981
days old
6 comments
5 participants
participants (5)
-
Andreas.Freiherr＠Vishay.com -
ard＠p850ug1.demon.co.uk -
ian_primus＠yahoo.com -
ipscone＠msdsite.com -
jss＠subatomix.com