Keyboard with cut cord Repair?

[flash9]

I am a newbie so, I am sorry if my questions have already been asked before, but I was unable to find them in my searches.

I purchased a NTC KB-6251 Keyboard, and the keyboard has a cut cord.
I have yet to receive it, but I was figuring I would just replace the old 5-Pin DIN or PS/2 stub with a new PS/2 cord.
I am pretty good with soldering and figured this would be an easy project.

In looking into the difference between an older 5-Pin DIN and PS/2 wiring I noticed there really was only minor differences, so even if this keyboard came with a 5-Pin DIN using a PS/2 cord should be an easy exchange.

My question come form the fact that in order to use this keyboard, I will need to then plug the PS/2 into an active PS/2 to USB adapter, but when I was looking at a wiring diagram for a USB plug it is the same four connections as the PS/2, so why not just use a USB cord?
What is in the active PS/2 to USB adapter, that let them drive older keyboards?

On a side note, I have been a visitor on this site since about the first of the year and it has been very helpful. I guess it my bad luck that the day I decided to join was the same day the site went up For Sale.

[Findecanor]

Welcome! Hopefully things won't change that much for years ahead. I doubt Webwit would sell to absolutely anyone.

USB and PS/2 are different protocols. An active adaptor acts as a host for the PS/2 keyboard and as a USB keyboard for the computer.
Now.. there are also some keyboards that support both PS/2 and USB, and those tend to use a single cable with a USB plug and then a passive adaptor to PS/2. Passive adaptors tend to be smaller and coloured green or purple depending on whether they are for PS/2 mouse or keyboard.
(I'm not sure how keyboards that support both protocols actually sense which one is connected)

While both interfaces have four wires, and both have wires for +5V and GND, PS/2 is a device-controlled protocol with a Clock line and a Data data line, but USB is a host-controlled with two differential data lines: D+ and D- (which is less vulnerable to interference at higher speeds). USB is much more complex and much faster.

When reverse-engineering the PS/2 pinout inside the keyboard, first find the GND line: It often has a black cable, and the traces on the PCB are most often wider and connected to the ground plane if there is one.
The +5V line often has a white cable and traces should go to every chip and to pull-up resistors if there are any.
The Clock and Data lines should go to the biggest chip, but to no other.
In my experience, getting the Clock and Data lines mixed up should be harmless: if it does not work, just swap them.

I'd recommend using a multimeter in "continuity mode", with probes to find where the traces go (unless they are very visible).

Do use an active adaptor! USB is hot-swappable but PS/2 isn't, and if you would short anything then you should hopefully only fry the adaptor and not your computer's USB port.

[Willy4876]

USB and PS/2 are very different protocols. USB in general is FAR more complex.

On a serial bus [USB], a single signal often includes address, control, data, and clock information.

(from this site).

Basically with USB there is a lot more data encoded into a single signal. There are other hardware differences between the two but I'm not going to go into that.

You can see some of the differences between the two protocols by looking at a waveform of the signals.

USB

Spoiler:

USB.jpg (84.33 KiB) Viewed 2179 times

PS/2

Spoiler:

PS2.jpg (26.42 KiB) Viewed 2179 times

Notice that with USB the two data signals are mirrors of eachother. This is what Findecanor was referring to when he said differential data lines. The signal seen by the USB device is actually difference in voltage between D+ and D-. With the PS/2 signal the two data lines are completely different signals (in the picture chan A is the clock and chan B is the Data).

[flash9]

Thank you for that clarifications, I kind of figured that there had to be more going on within the PS/2 to USB converters.
I know these are no longer being made, but am I correct in assuming these are one the preferred converters?
SANOXY PS2 to USB Adapter - https://www.sanoxy.com/product/adapters ... b-adapter/
I have five of these from an old job, and wonder what they are worth?

[Findecanor]

Indeed, the blue cubes used to be very well liked around here. I've heard that they would work with some vintage keyboards that other adaptors have difficulties with.

[XMIT]

For boards like this I'd get a Teensy Pro Micro, flash it with Soarer's Converter or TMK, hide it somewhere inside the case of the board and have a USB cable coming out.

[flash9]

XMIT - I like your idea, but for someone without a Soarer's Converter or a TMK, that option is not available.

[kmnov2017]

XMIT - I like your idea, but for someone without a Soarer's Converter or a TMK, that option is not available.

You have to make a converter on your own by buying a 3 euro promicro...

[XMIT]

Yeah. If it's just a PS2 converter you can crack it open and hide the converter inside the case to make things look a little cleaner. There should be a bunch of empty space inside the board, above the function keys.

Soarer's Converter and TMK are free-to-use keyboard protocol converters that you can flash onto a Pro Micro. This forum is full of details on these converters.

[Findecanor]

More correctly, TMK is a keyboard firmware framework, which includes a "protocol converter" ( = active adaptor).
The main info about the converter is here: https://geekhack.org/index.php?topic=103648.0 . You should be able to find not only source code but also binary files there, and links to ready-made adaptors with the firmware on.

Teensy 2.0 and Pro Micro are microcontroller boards that TMK and "Soarer's Converter" were made for.
There are many clones of the Pro Micro that are very cheap, but it was made for Arduino (a project for teaching microcontroller programming) so it has some unorthodox pin numbering.