A method of communications between a master unit and a slave unit of a supervisory control and data acquisition system is provided. The communications protocol has at least two bytes of data where the first byte (100) is an address byte containing address information for addressing the slave unit and the second byte (101) is a control byte containing an expansion bit and two address/type variable bits to be interpreted by the slave according to a rule set. In the rule set, the meaning of certain control bits (107) is related to the state of an expansion bit (103). The expanion bit defines the presence or absence of a third byte (102).