Nor-Cal SDC15 Controller - Manual (Page 176)

■ Communications procedures
The communications procedure is as follows:
(1) The instruction message is sent from the host device (master station) to one
SDC15 unit (slave station) to communicate with.
(2) The slave station receives the instruction message, and performs read or write
processing according to the content of the message.
(3) The slave station sends a message corresponding to the processing content as a
response message.
(4) The master station receives the response message.
Handling Precautions
It is not allowed to use two or more number of protocols together on
one and the same RS-485 transmission line such as CPL, MODBUS
ASCII format, and MODBUS RTU format.
8-2
Chapter 8. MODBUS COMMUNICATIONS FUNCTIONS