PURPOSE: To make connection between devices simple, simplify circuit configuration and make the device small and light by transmitting digital signals using three signal lines, i.e. a data transmission request signal line, a data signal line and a clock transmitting line.

CONSTITUTION: A data transmission request signal line 3 is connected from an output port P03 of a microprocessing device 15 in a device 2 to a device 1 through a protective resistance r3. In the device 1, the signal is sent to an input port Pi1 of a processing device 11 through a protective resistance r1. On the other hand, a data transmitting line 4 transmits two-way signals between the devices 1 and 2. For instance, when a signal is sent from the device 1 to 2, the signal is connected from an output port P01 to the device 2 through an open collector driver 12. In a clock transmission signal line 5, clock outputted from an output port P02 of the device 11 of the device 1 is connected to the device 2 through an open collector driver 13, and connected to an input port Pi3 of a device 15 through a pull-up resistance R3 and a protective resistance R5.