[coreboot] v3 parallel port console?

Carl-Daniel Hailfinger c-d.hailfinger.devel.2006 at gmx.net
Wed Sep 17 02:35:53 CEST 2008

On 17.09.2008 02:06, Peter Stuge wrote:
> 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.

Thanks for the info.

> 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.

Would checking connectivity with a multimeter suffice?

>> 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.)

One of the adapters in my collection supports keyboard and mouse
(tested). I didn't check the others (they all look the same).

>> 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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