I want to add an extra PIR detector to my PCX256 system. There's a PCX LCD keypad in the vicinity and it looks as though the keypad allows connection of a PIR input. That would save me having to run an extra cable back to the panel, but is it that simple? I don't understand how the panel would be able to see the new PIR.

My reading of the installation manual suggests that all I have to do is wire the PIR to the keypad and - hey presto - there it is, as input 243 on keypad 01.

Am I right, or have I misunderstood the purpose of the input on the keypad?

I think the pcx is based on the castle euro meridian, if so then yes that input is fully programmable as a zone, you will need to wire it an and enable/program the circuit as required.