The PC/XT used a clock/data protocol directly into an interrupt driven shift register which could handle 300 Kb+. The AT is microcoded and uses pseudo-RS-232 format  but with the bit windows timing relaxed to allow for for microcoding. The keyboard is dominate but the  buss direction can be reversed when the adapter pulls the clock line low and holds it.


Sent from my MetroPCS 4G LTE Android Device
-------- Original message --------From: karl@??? Date: 8/27/17 2:04 PM (GMT-06:00) To: dng@??? Subject: Re: [DNG] serial and ps2 ports.
Alessandro:
> On Sun, 27 Aug 2017 at 18:03:08 +0100
> Simon Hobson <linux@???> wrote:
> [...]
> > AFAIK the underlying protocol for the keyboard is the same (or near enough
> > for simple conversion) between the two connector formats to allow for easy
> > conversion between plugs.
>
>   They are, adaptors are purely mechanical.

PC/AT and PS/2 have the same protocol and electrical spec. except they
have different connectors. The protocol is bidirectional.

PC/XT has the same pinout and connector as PC/AT but not the
same protocol and they won't work together.
The protocol is keyboard -> pc only.

See page 16 of
  http://kbdbabel.sourceforge.net/doc/kbdbabel-vortrag-vcfe80-20070429.pdf
and
https://en.wikipedia.org/wiki/IBM_PC_keyboard
for details.

Regards,
/Karl Hammar

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