PURPOSE: To surely send monitor control information in a short time by providing a monitor control information area and I information representing the occurrence of a transmission fault on a transmission frame format and latching a prefix of monitor control information when the level of the received I information is logical 1.

CONSTITUTION: A transmission frame format consists of synchronizing information F, M information M, C information C. S information S, D information D, I information l, station address information A, frame class information D, information DATA in packet transmission and frame error check information FCS. The M information is sent and received only between mask stations, the C information is sent from a master station and received by all slave stations, the S information is sent from each slave station in OR processing, terminated by the master station and the D information is sent/received between opposite stations. When a reception fault takes place in each station, each station applies prefix latch to all five sets M-I of information and when the information I is received, each station applies prefix latch to all three sets M-S of information. Thus, the monitor control information is surely sent in a short time without requiring a communication protocol.