Re: Timeout: AT keyboard not present?

Maciej W. Rozycki (
Wed, 3 Jan 2001 16:39:11 +0100 (MET)

On Wed, 3 Jan 2001 wrote:

> (Ordinarily a key-up event gets scancode of key-down but with
> high-order bit set. In scancode set 1, a key-up event get the
> scancode of key-down prefixed by 0xf0. Since the high-order bit
> is masked here, this 0xf0 would show up as 0x70.
> Moreover, the key-up for a sequence like e0 71 is e0 f0 71,
> again what you see here.)
> How you got into scancode mode 1 I don't know
> (maybe by sending the command f0 01 to the keyboard).

It looks like untranslated mode 2 (i.e. the AT keyboard's mode). You
switch translation on or off by toggling a bit in the onboard 8042's
command byte.

PS/2-style keyboards start in mode 2 usually. An XT compatibility (bit 7
meaning release) is provided by the onboard 8042 translating codes. Some
systems (I think DEC OSF/1, for example) program keyboards to work in mode
3 (the PS/2 native mode) or possibly mode 1 (XT compatibility mode); for
these modes the 8042's translation has to disabled or the keyboard will be
next to useless. It's also meaningful to disable the translation in mode
2 -- you are presented with the AT interface then.

+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+        e-mail:, PGP key available        +

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