Keyboard PS/2 to Parallel converter
dave06a at dunfield.com
Wed Jan 9 09:46:26 CST 2008
>> If anyone is interested, I have a very small bit of 8051 code which reads an
>> attached PC keyboard, providing the scan codes over an RS-232 link. It also
>> supports commands to set LEDs on/off etc. I specifically avoided using the
>> P1 line in my design (full 8-bits parallel I/O available even on an 8031),
>> so these are free, making it very trivial to modify this to output parallel
>> instead of serial - it would also be very easy to add a translate table
>> and recognition of shift/ctrl modifiers so that it would output fully decided
> have you just the code or even the circuit PCB layout?
I've built up hand-wired prototypes, but I have not laid out a PCB.
The schematic is just an 8051 with power/crystal attached, or an 8031
with external ROM and address latch as per any 8051/8031 datasheet.
The PC keyboard clock and data lines are attached to the pins indicated
in the comments in my code. If you want RS-232 levels, you need a level
convertor (MAX232 etc.) on the 8051/31 RX/TX signals.
> And have you an idea to how to transform the circuit from serial to
Instead of polling for TX ready and writing the scancode to SBUF, just
write it to P1 (8-bit parallel output). You will also need a key-down
strobe, which you can implement via another pin, using key-down scancodes
to set, and key-up scancodes to clear.
Assuming you want ASCII instead of scancodes, you would need a translate
table (scancode to ASCII), and to recognize and track the state of the
SHIFT, CTRL keys and CAPS-LOCK state, either using alternate translate
tables (most flexible), or modifying the codes from one table accordingly.
Implementing rollover properly requires a bit more logic - once you have
processed a key-down, you must queue any other scancodes you receive until
you get the corresponding key-up, then process the queued codes after you
have released the strobe.
You can get fancier, implementing auto-repeat etc. if you like, the logic
is pretty straightforward (if key-down persists beyond the initial delay,
begin toggling the strobe at the repeat rate).
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
More information about the cctalk