The master sends a query message (question), and the subordinate returns a response message (response). When communication is normal, Device Address and Function Code are copied. When communication is abnormal (function code or data code is illegal), bit 7 (=80H) of Function Code is turned on, and the error code is set to Data Bytes.
Figure 1. Communication Message Structure
The message frame consists of the four message fields as shown in Figure 1. By adding the no-data time (T1: Start, End) of 3.5 characters to the beginning and end of the message data, the subordinate recognizes it as one message.