| 摘要 |
Surveillance of a site by independent sensors transmitting their signals to a central unit by HF facilities. According to the invention, the central unit normally equipped with an HF receiver 31-37 is used outside surveillance periods to account, in an up/down counter 50, for the successive openings and closings of the doors of the site to be monitored and, at the moment when monitoring starts, simple reading of the said up/down counter makes it possible to check that all the exits from the site have actually been closed. Application to anti-theft surveillance. <IMAGE>
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