COMMUNICATION SYSTEM FOR MULTI-DROP SYSTEM

PURPOSE:To attain polling in a short time by using a control means so as to apply monitoring for a no reply time to each slave station for only the time of reply information when a master station performs polling of each slave station. CONSTITUTION:A communication control module 2 starts polling based on a polling command from a master station 1. The switch 22 of a control section 21 is closed and a default is used till the replay time is counted to monitor a no reply time. However, the switch 22 is closed even once and the reply time of each slave station 5 is counted, then reply information decided based on the reply time is used to monitor the time by the operation of the program of the control section 21. At first all the power supply for each slave station 5, each MODEM 4, and a MODEM 3 is energized in advance and the operator closes the switch 22 in the module 2. When the control section 21 recognizes the switch 22 to be closed, the program for calculating the reply information is executed and the polling work is started.