Modbus Overview
This section is written for the person who will use the RT series RTU’s with Modicon Modbus protocols for communication in Modicon programmable controller applications. It describes the different types of Modbus registers supported. The RT Series RTU’s support Modbus, Slave RTU mode.
The Modbus protocol provides the internal standard that the Modicon controllers use for parsing messages. During communications on a Modbus network, the protocol determines how each controller will know its device address, recognize a message addressed to it, determine the kind of action to be taken, and extract any data or other information contained in the message.
All of the RT series RTU’s use a common Modbus communications protocol. All data is sent and received raw units.
Engineering Note: It may be necessary to force your PLC or Modbus master device to delay between Modbus packet transmissions. Some devices transmit without a break in data packets. Because of the nature of
Modbus Data: Some Modbus devices send data differently than others. This example shows Slave address 9, using function 3 and 16. It also shows he full data address of 40001. The RT series RTU’s do not send the full address. The Modbus protocol allows a system to assume the data address based on the function type. This is true of all Modbus functions for the RT series RTU’s.
09 03 9C 40 00 01 91 7E
09 10 9C 40 00 01 02 00 00 9E 54
09 = Slave 9
03 = Read Holding Registers
9C = Starting Address High
40 = Starting Address Low
00 = Number of Points
01 = Number of Points Low
7e = Check Sum
Manual Documentation Number: | 3 |
B&B Electronics Mfg Co Inc – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph
This is an example showing what data would look like for the RT series RTUs.
09 = Slave 9
03 = Read Holding Registers
00 = Starting Address High
00 = Starting Address Low
00 = Number of Points
01 = Number of Points Low
7e = Check Sum
4Manual Documentation Number:
B&B Electronics Mfg Co Inc – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph
