PURPOSE: To improve the operating efficiency of a user with a terminal equipment containing a communication port of a main body and a communication port connected as a peripheral device, by selecting automatically one of both communication ports according to the equipment constitution.

CONSTITUTION: A communication cartridge 2 contains a communication port 21 and is connected to a terminal equipment 4 via a cartridge interface 3. The equipment 4 contains a communication port 41. A host computer system 5 gives a command to the equipment 4 via a communication port 51 and the cartridge 2 to develop the user program of the equipment 4. A host computer system 6 transfers data with the user program via a communication port 61. An exclusive OR circuit 81 is provided to the inside 8 of the cartridge 2 to decide the attachment of the cartridge 2. A terminal 71 is connected to a terminal set at the cartridge option side 7 and having the fixed value. Then a terminal 92 is connected to a terminal set at the main body side 9 of the equipment 4. Thus the communication ports are automatically selected.