PURPOSE: To secure the simple connection to two host devices by using a switch circuit which switches the joint part of two systems connected to those host devices and connects both systems.

CONSTITUTION: A parallel interface circuit set between a host device and a printer contains connectors INFA(IN)2 and INFA(OUT)28 connected to one of two host devices, connectors INFB(IN)1 and INFB(OUT)27 connected to the other host device, and a switch 11 which decides connection to either one of both host devices. The output signal delivered from the switch 11 fixes the signal outputted from the INFB27 at 1 and 0 via OR circuits 19 and 20 and AND circuits 21 and 22 respectively when the switch 11 is turned off. Thus the logical connection is cut between the INFB27 and the host device. Then the other INFA28 only can have the logical connection to the host device set at the other side.