DIAGNOSTIC SYSTEM FOR FAULT

PURPOSE:To perform switching to a test mode without destroying any data frame on a LAN nor erasing any token by providing a circuit which generates timing for the disconnection of a port right after a token passes. CONSTITUTION:When a test request signal TR is sent from a master station 1 to a slave station 2, it enters an office through a selector 31 and is inputted to a decoder 32 and sent to a selector 33 as it is. The decoder 32 detects a token frame 42 behind a data frame 41 to generate a token detection signal, which is inputted to a delay register 35. The delay register 35 delays the token detection signal and generates the timing for disconnecting the port 21 right after the office outputs a token frame 42. Consequently, the port 21 is disconnected right after the token frame right behind the data frame is sent and a mode of intra-office sending-back diagnosis is entered.