Modbus communications procedure, Figure 4.1, Chapter 4 – Rockwell Automation 900 Temperature Controller Communications Functions - Series B User Manual
Page 59: Data format -1, Command frame -1, Chapter, Data format
4-1
Publication 900-UM004D-EN-E - July 2010
Chapter
4
Modbus Communications Procedure
Read this section if you are to communicate using the Modbus format.
Data Format
The data format complies with the Modbus (RTU) communications protocol,
so commands from the host computer and responses from the 900-TC8,
900-TC16, or 900-TC32 are contained in data blocks called frames.
The structure of the command and response frames is described below.
In the following explanations, hexadecimal values are expressed by adding the
prefix H’ before the number, e.g., H’02. Numbers and alphabetic characters
without the H’ prefix are ASCII characters.
Command Frame
When using RTU mode, start with a silent interval of at least 3.5 character
times and end with a silent interval of at least 3.5 character times.
Figure 4.1
CRC-16
1
1
Data
CRC-16 calculation range
2 bytes
Slave
address
Function
code