The IEC 60870-5-103 protocol was developed primarily for communication with digital protection relays using RS-232 or RS-485 serial ports. Each data category is sent using a specific ASDU (Application Service Data Unit). ASDUs 1 (one) and 2 (two) refer to digital points with time information. ASDUs 3 (three) and 9 (nine) are measurements, and ASDU 4 (four) indicates analog points with time information.

Inside each ASDU there is an identification of a point, digital or analog, referenced by a function that can be public or private, depending on the manufacturer, and an Information Type that indicates a data type. Please check a device's manual for a list of existing data types.