[sdiy] Keyboard scanner IC

[Nathan M. Reeves]

taken from the mirage list, Gordon was nice enough to suggest this.....

"No, it sounds like it's not doing much with the keys then.

The keyboard is scanned by U1, which is a mask-programmed 6502-variant.
They're used for the keypad in ESQ1s and the like, where they are
notorious for failing. Eep. Doesn't sound good, eh?

Pins 4 to 19 of U1 are pulsed in turn to strobe one side of the keyboard
matrix, and the returned keypresses come back through U2 and U5, which
are quad comparators. At this point I get fuzzy on the details, not
having played with a Mirage keyboard's insides for *quite some time*...

IIRC the keys are scanned in groups of four, with an "up" and a "down"
contact for each key. Velocity is determined by how long it takes for a
key to go from "up" through "neither" to "down". I think the uA339
comparators do something to clean up the signal.

Now, just a thought, but if one set of comparators isn't working, it
will either never see the "down" signal or never see the "up" signal.
Either way, it won't know what to do and won't emit any noise. You
should check with an oscilloscope to see if the inputs and outputs of U2
and U5 make sense. Here's what I'd do:

Use the 'scope to check for pulses on the outputs that go to the
keyboard (pins 4 to 19). Determine the cycle time to go through all
keys, and set the 'scope up so I can see two pulses from one of the
strobes.

Set the 'scope up to trigger on one of these pulses.

Next, look at the inputs to the uA339 comparators. Find out which one
changes when you press a key. You'll see *something* happen, which will
change depending on which key you press. You should see an input
disappear from one chip and reappear at the other.

Now check to see if the comparator outputs follow suit. If not, change
the comparator. If that still doesn't work, then U1 may be faulty."





Now on the schematics I  downloaded from tom (the original Mirage I  
believe), things are a bit off from what I see inside the Mirage DSK I  
am working over...


on mine

U1 SN74LS145N
U2 7406N
U3 7407N
U4 R6522AP
U5 WD1772-PH
U6 1380650001


on the schematics

U1 6500/11 MCU
U2 LM339N
U3 NE555
U4 6N138
U5 LM339N
U6 74LS145

So on my unit U6 is what U1 is on the schematic?  the 6502 variant MCU?

How do I setup my scope to see the pulse outputs on pins 4-19 on U6?

Do I attach the probe tip to the actual IC pin (4-19) and clip GND to  
the GND pin of the IC (looks like pin 40)?

This should be fun but I am very noob at using a scope to  
troubleshoot.....

Also my scope seems a tad bit under compensated and I can't seem to  
get it setup perfectly.....

the square wave on the probe tester on front of the tektronix looks  
more like a "sharkfin" wave 8^p

but that shouldn't cause concern should it?  a pulse is a pulse even  
if the waveform looks a bit off.....?

ANY advice or tips will be of GREAT help!

im excited to at least find the IC that is acting up on me  (i have a  
gut feeling its the MCU but need "proof" of this) and to get some  
practical scope use out of the experience as well!

THANKS TO EVERYONE WHO HAS ALREADY RESPONDED TO MY PLEAS FOR HELP!

Cheers,

Nate (scope noob) Reeves