Rockwell Automation 20-COMM-H RS485 HVAC Adapter FRN 2.xxx - Manual (Page 33)

20-COMM-H RS-485 HVAC Adapter User Manual
Publication 20COMM-UM009D-EN-P
Chapter 4
Using Modbus RTU
This chapter provides information about controlling a PowerFlex 7-Class
drive, setting its Reference, and accessing its parameters through
configurable objects when the Modbus RTU network protocol is selected.
Understanding Modbus RTU The Modbus RTU protocol is a messaging structure used to establish
master-slave communication between intelligent devices. The protocol
defines the format of the messages.
Messages from a master to a slave contain the address of the slave, a
function code defining the requested action, any data to be sent, and an
error-checking field. Messages from a slave to a master contain fields
confirming the action taken, any data to be returned, and an error-checking
field. If an error occurred in the receipt of the message or if the slave is
unable to perform the requested action, the slave will construct an error
message and send it as its response.
Modbus RTU can access a single address or multiple addresses
simultaneously, either reading or writing single-bit values or 16-bit values.
Modbus RTU Data Formats
Modbus data types are 1-bit and 16-bit values. Refer to Table 4.A .
Topic Page
Understanding Modbus RTU 4-1
Using the Modbus RTU Point Map for I/O 4-4
Accessing Drive Parameters 4-9
Using Broadcast Messages 4-12
TIP: Datalinks can also be used for accessing parameters. For information
about using Datalinks, refer to
Chapter 7 , Using Datalinks with All
Protocols .
Table 4.A Modbus RTU Data Formats
Modbus Type Description Reference
Coil Status 1-bit Discrete Output 0x
Input Status 1-bit Discrete Input 1x
Holding Register 16-bit Output Register 4x
Input Register 16-bit Input Register 3x