Abstract: In a printer that is connected to a host personal computer via an external recording device through parallel interfaces, a signal that has been received from the host personal computer is detected by an ASIC at specified sampling cycles and to allow an interrupt in case it is detected that this signal is a signal for instructing start of communication; in case the interrupt has been made while printing actions are being executed, a CPU notifies the host personal computer of a busy condition; in case no response is given from the host personal computer, the signal for instructing start of communication is invalidated. With this arrangement, it is made possible to accurately detect whether a received signal is correct or not and malfunctions owing to incorrect signals leaking out from the external recording device to the printer can be prevented.