| 摘要 |
PURPOSE:To discriminate a level of an input signal with a simple operation stably with high accuracy by devising the device such that a square wave test signal is outputted to a line driver from a selection circuit when a test request signal is entered from a test switch. CONSTITUTION:With a test switch 23 closed, a test request signal is outputted from an output terminal b1 of an input/output port 20 with a command from a CPU 1 and the signal is inputted to a selection terminal SEL of a multiplexer 21 as a selection signal. Thus, the multiplexer 21 outputs a square wave test signal inputted to an input terminal B1 and the square wave test signal is inputted to a line driver 3. When the line driver 3 is operated normally in this state and no fault takes place in the circuit part, the square wave signal whose voltage changes between -12V and +12V, whose period is almost one minute and whose duty factor is 50% is outputted at an output terminal of the line driver 3. Thus, it is possible to discriminate that a level of the input signal to a MODEM 6 is a prescribed level with a simple operation. |
