Cutting a keyboard pcb

[Bjack795]

Hi all!
I've a strange question on the keyboard pcbs, can you technically "cut" a part of it keeping the other keys working? what I mean is: are the keys isolated on the circuit or if you cut an angle of the pcb the other keys die?
I know that this depends on the circuit but I mean "normally" are they isolated?

I want to reduce the size of an old 100% pcb without buying a custom smaller board, is this possible?

I know this is weird, please don't kill me 

[Findecanor]

In general, yes. A regular keyboard matrix made out of mechanical switches can be cut: it is electrically the same as if the keys are never pressed.
I and several others have done that to some inexpensive vintage keyboards, making tenkeyless or smaller keyboards out of full-size keyboards.

Whether it is feasible or not to do with your keyboard PCB depends on where the traces are on that particular PCB.
If you are very lucky, you only have to make straight cut that won't cross any traces.  If you are very unlucky, there are electronic components other than switches and diodes on the part that you want to cut off, and then the mod might be too difficult. Most mods like this do cut a few traces in the keyboard matrix only, that would have to be reconnected by soldering wires.

Keyboard PCBs are almost always in one or two layers: the outer layers, thus making it possible to see and follow the traces. Only by doing that will you be able to assess whether it would be worth it or not.

[Bjack795]

Perfect, thank you!

[Bjack795]

Another question, I want to recover an old keyboard for this project (I haven't found it yet) but can the old keyboards be reprogrammed?

[suicidal_orange]

Hardly any commercially available keyboards can be reprogrammed and definitely not old ones.

You might be lucky and find an old board which has a separate controller PCB and want to keep (not cut off) the part with the connector which you could then wire up to a Teensy but then you'll have to work out the matrix, unless it's labelled...

Why are you thinking of doing this?  Hopefully not because it's cheap because it probably won't be after you get a custom case cut (the mounting holes in a random board won't fit a standard 60% case, for example)

[Findecanor]

I have never heard of any controller being reprogrammed, but some people have removed the controller chip and replaced it with wires to their own controller.
This usually requires lots of wires: one for each row and column, and also the indicator LEDs. I suggest using ribbon cables as much as possible to avoid something looking like a rat's nest. Most people modding keyboards with plate-mounted switches even remove the PCB completely and replace it with "direct wiring": diodes and wires soldered directly to switches.

If the keyboard is not too unusual, however, you could possibly create a "protocol converter" (active adapter) that converts its protocol to USB ... and program your custom stuff into its firmware. That is often much easier hardware-wise.
There are several free firmwares available for microcontroller boards based on the ATmega32u4 microcontroller: from e.g. AT and PS/2, XT, IBM terminal, first Macintosh, Apple ADB, Wyse and Next ... even USB to USB.
You will find these in the Making Stuff Together! subforum. (This thread should have been in that subforum in the first place ... )
The most common firmware is Hasu's "TMK" firmware collection which includes firmwares for both keyboards and converters.
The most common microcontroller boards are the Teensy 2.0 and the Arduino Pro Micro. The latter is open hardware so there are lots of cheap clones on eBay.

[Bjack795]

Thanks to all,
So I have already the need to build a custom case for my project so this is not the problem and this mod could make me spend 150€ less.
Since it is difficult to reprogram it and since I'm writing the OS of the project (E-ink typewriter) I will simply reprogram the ASCII table of the code, even though it will be a pain.

Ps. I'm sorry for the wrong section

[ErgoMacros]

In case you don't know, keyboards almost never (for sure never for PS2 and USB) send ASCII down the cable.
They send "keyboard scan codes." I mention this because you're doing the OS too.

The modifiers (Shift, Control,...) of course don't generate ASCII codes. The keyboard sends a key-down, then key-up for each position. The OS watches all that stuff, and "knowing" which international keyboard is attached sends the right character to the app. (Apps can also detect shift key down as well.)

If you want to ever use a different keyboard on the project you'll want to design your keyboard to output scan codes.

all the best.

[Bjack795]

I have an USB host board reading the keyboard and sending me out the ASCII codes, I can't touch it because it's too long, I will change the correspondence directly in the OS