PURPOSE: To easily reset the hang-up of a converting device on a data processor side by resetting the converting device in response to the supply of no next flow control switching signal in a specific time.

CONSTITUTION: The converting device 3 waits until an XOFF signal is received, starts time clocking action, and decides whether or not an XON signal has been received. When it is decided that the XON signal has not been received, it is decided whether the clocked time has reached a specific reference time and when not, a decision is made again. When it is decided that the specific reference time has been reached, on the other hand, a fault detection signal indicating that a fault occurs to a terminal device 5 is generated and sent out to the data processor 2, and a reset command from the data processor 2 is expected; and an PS232C control task, a buffer, etc., are eliminated, and an RS232C control task is newly restarted to perform the 1st process again.