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XT/AT autoswitching keyboard?

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First post, by jwt27

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I have a keyboard here, a Compaq Enhanced II. Looks just like any other keyboard, rubber dome switches, enhanced layout, PS/2 plug, nothing special.

On the inside it's a bit more interesting. It's built in the same style as a Model M, with the keyswitches sandwiched between steel and plastic plates. The cord is also detachable and shares the same AMP connector with the Model M. And like the Model M, this keyboard was also manufactured in the UK.

Now comes the interesting bit: it can detect whether it's connected to a PC/XT or AT/PS2 pc, and automatically switch between the protocols. Now I've heard of keyboards that have a switch on the bottom to select the protocol, but an autoswitching keyboard is something new to me. I was wondering, was this a common feature in the transition phase between the XT and AT, or is it something special?

Reply 1 of 1, by Jepael

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I recall I had a keyboard that could autodetect the protocol it should use.
Before that, I also had a keyboard with protocol select switch. Later keyboards I had were AT only, AT or USB, or USB only.

I think the autodetecting keyboard it defaulted to XT protocol, but used AT protocol when connected to a PC which uses AT protocol. I never analyzed thoroughly how it detects it, but I recall seeing different default protocols from different keyboards. I wanted to make a DOS game that would use two keyboards (and two mice), and the second keyboard was connected with an adapter to the parallel port.

I also recall AT keyboard hosts may want to send out something to keyboard at boot, to reset it or read ID or something, so this might be a way for the keyboard to detect it. Other approach is for the keyboard to use XT protocol to send out the power-up test OK (BAT) byte, and the host won't understand that and sends the keyboard a command in AT protocol to resend information, so the keyboard can then switch to AT protocol, because XT hosts are not able to send information.

Some motherboard keyboard controllers might have been able to work with both XT and AT keyboards too, but I don't know.

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