| 摘要 |
<p>In a fire-alarm system in which a plurality of terminal units (T1,T2...) are connected to a fire receiver via a main signal line, the fire receiver (RE) polls each terminal unit, and the terminal unit called by the polling sends a current signal back to the fire receiver, a voltage signal receiving circuit (20) having a DC signal component blocking means is provided in each terminal unit for cutting a DC signal component of a polling signal to detect only an AC signal component of the polling signal, and a current signal transmitting circuit (40) having a constant-current circuit is provided so that the constant-current circuit is turned on/off to deliver the return signal. Therefore, a threshold used for signal discrimination in the voltage signal receiving circuit can be easily set. The current consumed by the terminal unit when it is outputting the current signal becomes constant regardless of the number of the terminal units connected, resulting in that the terminal unit will not wastefully consume an extra current. <IMAGE></p> |
