GB1349884A - Information retrieval systems - Google Patents

Abstract

1349884 Selective signalling HONEYWELL Inc 19 April 1971 [25 Feb 1970] 20823/71 Heading G4H In a system having a central station and a plurality of groups of remote stations, each group has connecting means which sequentially connects the stations of the group to the central station, the groups transmitting simultaneously through respective channels, whereby upon transmission of a scanning signal (being at least one bit of an address signal sent) by the central station the remote stations in each group are sequentially scanned by the respective connecting means and the information relating to their condition is transmitted thereby to the central station where it is stored. In general, a given channel has more than one group associated with it, the scanning signal being accompanied by an address to select the group to be scanned. Each station is a switch in an air-conditioning monitoring system and the information sent to the central station about it including an indication of the data type, a status bit indicating open or closed, a bit indicating whether the status has changed since the last scan, a bit indicating whether it is the last station in the group, and a bit indicating whether it is the last station in the channel. The stored information at the central station is processed to control a printer. All messages to the central station are sent twice and compared for error detection, inequality causing up to three retransmissions from the erroneous channel, an error indication being given if error-free transmission is not obtained during this. Failure to receive a message over a given channel similarly causes repeated requests for transmission, &c.