PURPOSE: To allow a controller to be set automatically so that a communication procedure of an on-line terminal system is the same as a communication procedure of a data communication network by discriminating the communication procedure of the data communication network.

CONSTITUTION: An on-line terminal power supply controller 3 outputs a discrimination signal used to discriminate whether or not a data communication network uses, e.g. a packet mode communication procedure to a host computer being a heart of a data communication network via a MODEM 1. When a reply to the discrimination signal is sent to the on-line terminal power supply controller 3 from the host computer, the on-line terminal power supply controller 3 discriminates it that the data communication network uses the packet mode communication protocol. On the other hand, when no reply comes, the on-line terminal power supply controller 3 discriminates it that the data communication network uses, e.g. a private line communication protocol. Then the on-line terminal power supply controller 3 sets automatically the communication protocol of the on-line terminal system so as to be in matching with the communication protocol of the data communication network.