PURPOSE: To allow the plotter to discriminate a data bit length of data and a method for parity check sent from a host computer completely automatically when the plotter receives the data from the host computer by having only to implement the operation of mere connection of the plotter and the host computer with a cable for data communication without the need for any expert intelligence.

CONSTITUTION: Sent data in which the sum of '1s' in data bits from 1st to 7th bits is an even number and sent data in which the sum of '1s' in data bits from 1st to 7th bits is an odd number are compared and the data bit length and the method of parity check are discriminated based on the value of 8-th bit or the value of 8-th and 9-th bits of the data of even and odd numbers.