Assuming same power supply the 0v is the same
tr is just that and isto inform the pnal of tamper from the bell. Diff bells report/detect different things
testing should be clear when in normal state, should be in alarm when attacked
Put your meter on ac volts
Pos probe to 12v dc
Neg probe to good mains earth
Check reading with mains on and mains off
Ensure meter isn't in current probe position