I bought a couple of these 16-button alphanumeric keypads a while ago, to use for various projects. They were dirt cheap with free shipping so the lack of a fancy label on them didn't worry me too much. If they worked they worked, and if they didn't - well, hey, not a big loss.
What I didn't take into consideration, was the fact that they didn't come with any documentation. At all. I had no clue what the pinouts were. Searching was useless, and the few similarly looking datasheets I found didn't match at all.
Since I got a new pair of probes for my multimeter today, I had a excuse to investigate this case further.
Finally a proper pair of probes, rated up to 600V/10A.
"Reverse engineering" isn't the proper word for this job, I think. There's no engineering involved, it's more... figuring out how it works. Anyway, time to figure out how this thing works, which should be a very simple job. As expected, opening the keypad revealed a single-sided PCB - although with a strange design. I guess the jumpers are inevitable on one side, but traces leading nowhere (broken by drilled holes) made no sense to me. You should be able to see a few of them vaguely on the image below:
Strange design, but still simple to follow.
I'm guessing multiple boards are produced simultaneously, and e-testing is done on the entire panel before separating and traces are cut by drilling holes.
Two boards, similar - but different.
...but wait. The other PCB is mounted upside down. I don't know wether my recently acquired mapping is correct, or as incorrect as it gets. I opened the other one and confirmed that both PCB's are identical. Must've been one assembly worker in China having a bad day, I guess.
Either way, if you have one of these, you should be able to figure it out from this map:
COL1 COL2 COL3 COL4
ROW1 (1) (2) (3) (A)
ROW2 (4) (5) (6) (B)
ROW3 (7) (8) (9) (C)
ROW4 (*) (0) (#) (D)
Column 1 = pin 8
Column 2 = pin 1
Column 3 = pin 2
Column 4 = pin 3
Row 1 = pin 4
Row 2 = pin 5
Row 3 = pin 6
Row 4 = pin 7
When you notice the numbering, it makes you go all "ahh, but why didn't.. since.. jumpers.. oh, fuck it."
Scanning each row and column sequentially reveals if one or more buttons are pushed. It's worth mentioning that I experienced the internal resistance of these devices to be up to 70-80 ohms, due to the rubber contact pad.