How can I connect a DIN plug keyboard to USB?

Question

I have a wonderful IBM Model M keyboard, which has the best key action of all keyboards I ever used. The problem is it has the big round 5-pin DIN connector used in AT PCs.

I have some PS/2 to USB converters, and I know they are very 'moody', often only working with keyboards they were shipped with, and sometimes not even that. I do have DIN to PS/2 connector, but - expectably - the double conversion it doesn't work with these PS/2 to USB converters.

How can I use my 'retro' keyboard with modern PCs without PS/2?

Answer 1

If the IBM Model M keyboard is one of the ones that uses the AT protocol, you may be able to use a passive 5-pin to PS/2 adapter, chained with an active PS/2 to USB converter such as the Belkin F5U119.

If it uses another protocol (such as XT or 3270 terminal) you will need a custom-made active converter -- https://deskthority.net/wiki/Converter lists several, of which the best known is probably "Soarer's".

Answer 2

The easiest (PnP) approach is definely using two adapters. The first one should be a USB to PS/2 (miniDIN) adapter; This one needs to be an active adapter, such as this one:

Avoid using weird smaller adapters like the one below - they probably won't work because they're designed for motherboards that have a PS/2 host, which allows them to be passive (motherboard will be in charge of changing D+ and D- from the USB port to Clock and Data signal from PS/2). If you have one lying around, you can always try it - your PC will be fine even if the keyboard won't work with this one.

The second adapter should be a passive one; You can either make one yourself (working schematic on the last picture) or buy one like the picture below online.

One last thing; If you're going to make an adapter yourself, you should not connect the same colored cables from the DIN cable to the same colored cables from the miniDIN cable; Always verify what cables you're connecting to what pins and double check if the polarization is OK (ie. that you haven't connected + from the miniDIN to a - on the DIN)

Answer 3

There are two aspects to this. The cable for the physical connection and the driver for the communications protocol.

I suspect with adaptor upon adaptor you are either losing voltage and therefore connectivity or the protocol is getting confused.

Your core problem will lie in the communications protocols. There is a defined USB protocol to which peripherals such as keyboards must comply. The old IBM keyboard clearly doesn't. The host computer will interrogate a USB device using a Setup Token Packet. A USB keyboard will respond, the XT keyboard will not.

PS/2 to USB converters, and other USB converters, usually have a small embedded processor that handles the protocol.

So:
Writing a USB driver on your new PC to bypass the USB protocol is probably impossible as the USB ports are likely to be driven by a dedicated chip.

Cutting the PS/2 connector off your USB-PS/2 adaptor and soldering the wires directly to the DIN plug - in the hope that the voltage loss is reduced and so the flakiness you describe goes away is very risky. Also, the protocol chip may be in the PS/2 plug!
Although the pinouts are not too bad. The IBM DIN and PS/2 sockets have pins as below:

1: Clock
2: Data
3: Reserved (in practice, usually a reset line)
4: Ground
5: +5V

I would suggest: Use a USB-RS232 adaptor and wire the DIN plug to a DB25 or DB9 RS232. The keyboard will then appear to the PC as being on a standard COM port. You then write a keyboard driver for your new PC that converts the data stream from the old keyboard. You may be able to then redirect COM1 to STDIN.

The protocol is summarised as (taken from this description):

Keyboard - PC

When the keyboard has a byte to send to the computer (a keystroke), it shifts 9 bits out to the data line (RxD) with nine clock pulses on the CLK line.
The data format is 1 start bit, followed by 8 data bits.
The baud rate is roughly 2000 bits per second.
The byte sent represents the scan code of the pressed key or a response to a command.

PC to Keyboard:

Commands

ED <byte> Set LEDs depending on byte
          bit 0 is Scroll lock
          bit 1 is Num lock
          bit 2 is Caps lock

EE          Echo EE
F0 <mode>   Select mode 1, 2 or 3
F2          Send keyboard I.D.
F3 <byte>   Set repeat delay and rate
            byte is: 0ddbbaaa
                 delay is (dd+1)*250 msec
                 rate is (8+aaa)*2^bb*4 msec

F4          Clear buffer
F5          Restore default settings and wait for enable
F6          Restore default settings
FA          Acknowledge
FE          Error- please retransmit
FF          Reset keyboard

I have heard of people trying to achieve this feat by putting a Raspberry Pi or Arduino between the keyboard and PC as a protocol converter. But I don't know of any "How To" guide for this.

The upshot of this is that it isn't trivial. Good luck.

Answer 4

have some PS/2 to USB converters, and I know they are very 'moody', often only working with keyboards they were shipped with, and sometimes not even that.

There are two very different types of converters you might be dealing with in this situation.

Wire Converter

One simply connects the wires, changing the connection from a mini-DIN to a USB connector, with no protocol changes at all. These work only with modern keyboards that know how to do both AT (more often called "PS/2") and USB keyboard protocols, and automatically switch depending on what they're plugged into. In other words, what's happening with these is that the protocols is already USB at the PS/2 connector, and the adapter is just changing the connector shape. They typically look like this:

Typically if an adapter is supplied with a keyboard, it will be one of these "wired through" connectors because it's much cheaper to build a keyboard that speaks both protocols than it is to build a keyboard that speaks just one and an entirely separate electronic system to convert the protocols.

PS/2 to USB Converter

The second type talks the AT protocol on one side and the USB protocol on the other, being essentially itself a small computer that converts between the two. These will often look a little bit larger, but not always:

What You Need

Because your old AT keyboard talks the AT (PS/2) protocol only, you must use the second type of adapter. Anything that came with a keyboard is most likely the first type of adapter. Anything without a second connector for a mouse is probably the first type of adapter, unless you have some evidence otherwise.

You'll also probably need a connector to convert from your larger DIN connector used by AT keyboards to the smaller mini-DIN connector used by PS/2 keyboards, but the protocol is exactly the same for both AT and PS/2, so these will simply be wired through.