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Manual Documentation Number: RT-12 series-4006m 3
B&B Electronics Mfg Co Inc – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104 – www.bb-elec.com
B&B Electronics – Westlink Commercial Park – Oranmore, Galway, Ireland – Ph +353 91-792444 – Fax +353 91-792445 – www.bb-europe.com
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 half-duplex radio you may need to
configure or program your device with a 30-50 mS delay between packets and a 1.5-
2.0 second timeout.
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 -/ 40001
00 = Number of Points High-\
01 = Number of Points Low -/ Read 1 Point
91 = Check Sum
7e = Check Sum
4 Manual Documentation Number: RT-12 series-4006m
B&B Electronics Mfg Co Inc – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104 – www.bb-elec.com
B&B Electronics – Westlink Commercial Park – Oranmore, Galway, Ireland – Ph +353 91-792444 – Fax +353 91-792445 – www.bb-europe.com
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 -/ 40001
00 = Number of Points High-\
01 = Number of Points Low -/ Read 1 Point
91 = Check Sum
7e = Check Sum