On 2010-07-29 13:29, alan.b.pearce@stfc.ac.uk wrote:
>> I'm working on a project based on this
>>
>> http://home.clear.net.nz/pages/joecolquitt/pic_keyboard.html
>>
>> The intention is to monitor/capture/transmit keypresses of the
>> PC keyboard, using a PIC circuit between the keyboard and the
>> PC. Although it can be added in parallel
>>
>> A problem has arisen which has me stumped. The keyboard
>> freezes when the PIC transmits a character down to the PC.
>> The PC still recognises and displays any further characters
>> sent by the PIC, so the clock and data lines are still working
>>
>> As far as I can tell, according to the pdf linked to on the page,
>> the PIC transmission is within timing specs (as evidenced by
>> the PC recognising characters sent). On occassion I've had
>> the PIC emulate the timing exactly but the keyboard still stops
>>
>> As I understand it, PC keyboard outputs are open-collector
>> or open-drain with pullups to Vcc. My grounding transistors
>> are in parallel with those, which I wouldn't have thought to be
>> a problem. I've also tried disconnecting the keyboard during
>> a PIC transmission using 4066 switches but no luck
>>
>> I see no glitches on data, clock or Vcc, and there's no sequence
>> which would put the keyboard in a state to receive commands.
>> All I see is a character going to the PC from the PIC
>>
>> Any thoughts ? When it's sorted out I'll put the relevant parts up
>> as a project/reference on PIClist
>
> I suspect the keyboard is seeing the characters you are sending as
> commands to the keyboard, but as it doesn't know what to do with them it
> goes into dumb mode.
>
> But then I thought this was how barcode scanners and the like worked, so
> that doesn't make sense either ...

Barcode readers designed to be used between a PC keyboard
and a PC, uses a "Wedge". Now, I do not think that thing
simply hard-wires the in and out connectors as in this case.
I'm pretty certain that it contains some electronics to isolate
interfaces.

You can't see/know without opening one, of course.
--=20
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