Alx

[james.wilson]

Id was/is clever but basic and doesn't handle any interference. 

It not data as such

In 100ms the panel would pulse between 2.85v to 6.5v 

1 pulse for biscuit 1 10 pulses for biscuit 10 and so on. Each biscuit counts the pulses and draws a different current if open or closed after its pulse count has occured

After all 30 have done this the panel rises the line to 11.25v which resets the counters and tells the biscuits to start counting again. All in 100ms. If a biscuit reports open for 3 scans (300ms) the panel then reports alarm. 

Later panel had this programmable though

 

You can imagine how any sort of interference would cock up this scanning