Keyboard Converting Guide

The Keyboard Converting Guide is designed to help produce a tailored keyboard adapting or converting guide based on your answers to a series of questions asked to identify the keyboard and its connectivity. Please answer each question below with the answers that best apply to your keyboard.

Contents

Details

The combination of a 5-pin 180° DIN plug, IBM PC Mode 2 protocol and IBM scancode set 2 was used as the IBM Personal Computer AT keyboard interface. It is nicknamed the AT keyboard interface after the IBM Personal Computer AT, used to contrast it with the earlier XT keyboard interface that makes use of the same physical plug and pinout but are not data-compatible with each other.

The use of IBM PC Mode 2 protocol means this interface is related to the Display Station (terminal) and Enhanced (PS/2) keyboard interfaces, which all share the same carrier serial format but differ in exact scancode and command set usage. An Enhanced variant keyboard is natively compatible with an AT host, and an AT variant keyboard can operate with effectively all PS/2 hosts so long as no software requires a scancode set change. AT variant and Display Station variant keyboards are typically not plug-and-play compatible with each other's hosts without specific software support (which is rare anyway).

Applicable keyboards

The following IBM and family keyboards are known to use this keyboard interface:

*Despite its name, the IBM PC/XT Enhanced Keyboard natively supports being used over an AT keyboard interface

Converting to USB

AT to USB almost always requires active conversion. The AT keyboard interface was already a decade old by the advent of USB, and by that point, the PS/2 interface had largely replaced AT anyway. Some PS/2 keyboards (which can also function as AT keyboards) made since the advent of USB may offer passive PS/2 to USB support, but this is not to be assumed for AT keyboards. The majority of AT keyboards were made well before USB was conceived anyway, but whilst a limited number of such keyboards were still being made in the late '90s, it is unknown if they might support USB as well. For safety, do not assume even late AT keyboards can support USB.

There are two approaches for AT to USB conversion. Firstly, you can take advantage of AT-PS/2 cross-compatibility. Active PS/2 to USB converters are commercially available and generally affordable, so a cheap passive AT to PS/2 adapter used in conjunction with one should provide a cost-effective USB solution. They typically have both keyboard and mouse sockets present, but the latter can be left unused. The following are known active PS/2 to USB converter choices.

Secondly, the common Soarer's Converter or Vial-QMK ibmpc_usb firmware both support AT keyboards and can be used for a more costly but feature-rich converter. Notably, they both offer hardware-side layer, macro or remap creation. Both will advertise XT and AT support concurrently - that is normal. The following are currently available pre-made converters that are flashed with either firmware:

Converting to PS/2

AT to PS/2 needs only passive adaption. AT keyboards typically work on PS/2 hosts without fuss since their protocol is very similar and PS/2 by default uses the same scancode set that AT is limited to. Such adapters are electrically simple and often generic, so there is no specific brand people may try to target. So long as the adapter looks like what is pictured above (note the number, placement and direction of the pins) and is labelled like "AT to PS/2", "DIN to Mini-DIN", etc. it should suffice. You should be able to find such adapters on eBay, AliExpress, etc.

Still, please be advised that many PS/2 motherboards will not support hot-swapping the keyboard (connecting and disconnecting whilst the PC is turned on) and will require the system to be turned off before the keyboard can be safely connected and recognised. If you passively adapt an AT keyboard and connect it through a PS/2 port and it does not immediately work, simply try restarting your PC with it connected.

Converting to XT

AT to XT generally requires active conversion. XT and AT keyboard interfaces may use the same plug, but they are not normally compatible with each other. Some third-party AT-compatible keyboards had a switch to change between XT and AT compatibility, but this was not the case for IBM and family keyboards. If you have a third-party keyboard, please check for such a switch. If it was present, it could be labelled "XT/AT", "X/A" or "8088/80286".

Otherwise, active converter solutions for AT to XT indeed exist, but options are limited and prices for such may reflect that. The following three options work but they offer only a PS/2 input port though, so a passive AT to PS/2 adapter is also required.

Sources

ASK. Admiral Shark's Keyboards original content. License/note: CC BY-NC-SA 4.0.