The original install was a 2 Zone Bently system installed 30+ years (cabled during construction). Over time all the panel LEDs have blown, and recently during some decoration work tradesmen removed the PA causing the panel to go into alarm. At this stage, we realised that the bell was no longer working. Some testing later and there is no supply for the bell from the Mosfet. Hence a new panel.
The old install had a PIR and door in series for INT and TA, but on an 8 core cable so it seemed an obvious choice to use the spare pair for the second zone, except as above shorting the pair at the far end gave no continuity. (but i can prove the orange is good by shorting to the TA pair)
I don't understand the reason why (perhaps you guys might know?) but for this circuit they ran an 8 core from the panel but only a 6 core comes out of the coving to the PIR, with a junction behind the coving. The 8 core then continues behind the coving round the room before popping into a small loft where the 8 core is junctioned onto 6 core for the door. The black and red is disconnected onwards from the PIR which makes sense, as there was no need to run power to the door. Why would they have used 8 core - maybe they had loads on the van, but i cant see any other reason to have done so?
I'm a comms guy, so I have tone tracers and a TDR meter which shows that the "break" is about 20cm closer to the panel than the pair used for the PIR power - My guess is therefore that the break is at that first junction which is not accessible ?
The G4 zones don't seem to have common pins - there are a few kohms between the first pin on each zone, and more between the second pins, so i'm guessing all go back to the micro rather than a simple pullup/pulldown arrangement.