English
By using two resistance values, the idle state and the activation of the detector are transmitted. The idle state is determined by the basic value of resistance, and doubling this value results in activation. A short circuit or disconnecting the loop is considered as a sabotage of the loop or opening the cover of the detector.
The ALARM contacts (the output is open on the detector activation) and TAMPER (the conact is switched off when there is a sabotage attempt) are always NC - i.e. the switched contact represents the idle state.
The idle state is always transmitted into the system as a log. "0" (though it actually means a switched contact of the detector!).
The loop resistance:
|
The state |
A typical value |
Permitted range |
ALARM contact |
TAMPER contact |
|
Sabotage |
0 |
0 ÷ 100 Ω |
The loop short circuit |
|
|
Idle state |
1k1 |
870 ÷ 1,250 Ω |
switched |
switched |
|
Activation |
2k2 |
1,750 ÷ 2,500 Ω |
open |
switched |
|
Sabotage |
∞ |
> 7 kΩ |
x |
open |
Fig. 1. The basic connection of the circuit of double balanced loop
Notes:
-
The loop resistance is measured by the relevant module input, and the module evaluates the measured value and transmits two binary variables, ALARM (the idle value corresponds to log "0") and TAMPER (the idle value corresponds to log "0").
-
The values of the resistors have a tolerance band of approx. 10 %, to avoid a problem of bad evaluation resulting from a higher tolerance value of resistance, or fluctuation of the resistance value, e.g. due to temperature.