[coreboot] v3 parallel port console?

Peter Stuge peter at stuge.se
Wed Sep 17 02:06:19 CEST 2008

Carl-Daniel Hailfinger wrote:
> All of them have PS/2. However, some only have one PS/2 port which
> is either a pure keyboard or a combined keyboard/mouse connector.
> The latter may pose challenges regarding pinouts and/or electrical
> interfaces (multiplexing?).

PS/2 uses four signals; power, ground, clock and data. Dual-purpose
mini-DINs (e.g. page 10 in http://www.pcengines.ch/schema/alix1c.pdf)
use the two last pins in a 6-pin connector for the second clock and
data pair.

Y-cables are simple splitters.

The 6-pin connector is backwards compatible with 4-pin plugs, so
plugging a keyboard directly into it will work without a splitter.

Carl-Daniel Hailfinger wrote:
> Short version: Some keyboards are dual USB+PS/2 keyboards and have
> an USB connector by default. They often come with a pure passive
> adapter which will convert the USB pinout to a PS/2 pinout.
> http://us.st12.yimg.com/us.st.yimg.com/I/directron_2019_60686738 is an
> example image of such a female USB -> male PS/2 adapter.

The image shows a mouse adapter.

Did you actually take one apart to verify that it is nothing more
than a passive adapter?

As you know, USB signals are much faster than PS/2, and I would be
very surprised to see electronics in keyboards clever enough to
support both over the same wires. That's not a simple trick.

I would however not be at all surprised to learn that those green and
purple adapters actually have electronics in them to translate
between USB and PS/2.

> Some time ago, a few pranksters (myself included) suggested to use
> such an adapter to plug USB flash drives into legacy PCs without
> USB ports. Of course that wouldn't work due to the passive nature
> of the adapter (electrical and protocol mismatch).

I would instead guess that it will not work because the electronics
in the adapter was designed and implemented to support one specific
type of USB HID device. (Keyboard, or mouse, but not both. Oh, and if
it was a passive adapter there would be no difference between
keyboard and mouse adapters.)

> The PS/2 port is generally seen as a pure input "device" and using
> it for complex output (i.e. not just switching keyboard lights)
> will certainly baffle lots of people and confuse those "of course
> that won't work" experts.

Those experts might be well served by reading up on the PS/2
protocol. The signalling is very much bidirectional between the
keyboard controller on the mainboard, and the microcontroller in the
actual keyboard itself.


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