To greatly shorten polling time for confirming the presence or absence of theft and to reduce the risk of theft in the other slave machine during a polling period by transmitting a response signal with which the discrimination signal of the presence or absence of theft is overlapped when the last slave machine receives a confirmation signal, overlapping the discrimination signals of the presence or absence of theft with the response signals and sequentially transferring them to a master machine by means of the other slave machines.
P-pieces of cables 31...3P are connected to a master machine 1. N-pieces of slaver machines 21-1, 21-2...21-N are cascade-connected to the first cable 31. The master machine 1 periodically transmits confirmation signals to the slave machines 2. The confirmation signals are sequentially transferred to the subsequent slave machines 2 by the slave machine 2 and they are received by the last connected slave machine 2 (slave machine 21-N, for example). The last connected slave machine 2 outputs a response signal and the response signal is transferred to the master machine 1 by the halfway connected slave machine 2. The respective slave machines 2 overlap the discrimination signals of the presence or absence of theft with the response signals.
TAKAI DAISUKE
NAKAMURA HITOSHI
