© 2020 Van Putten Instruments BV | MAN-VP-V-UK | Revision: 2000 | Date: 29-7-2020
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9
Apendix
9.1
Modbus networks
Introduction to Modbus
Modbus is a messaging structure developed by Modicon in 1979. It's purpose is master-slave/
clientserver communication between intelligent devices. It is a de facto standard, truly open and the
most widely used network protocol in the industrial manufacturing environment. The Modbus protocol
provides an industry standard method that Modbus devices use for parsing messages. For more
information see modbus.org.
How does it work?
Modbus communication is called "Master-slave" communication: The master can initiate
transactions (called queries). The slaves respond to the master, take the action requested in the
query. A slave is any peripheral device (I/O transducer, valve, network drive, or other measuring
device) which processes information and sends its output to the master. Masters can address
individual slaves, or can initiate a broadcast message to all slaves. Slaves return a response to all
queries addressed to them individually, but do not respond to broadcast queries.
Register map
Modbus devices usually include a Register Map (Point Map). You should refer to the register map for
your device to gain a better understanding of its operation. The available options and registers of the
point map are device-dependent. A simple sensor might have only one register, while a multi
parameter sensor might have ten or more registers.
Communication modes
Standard Modbus networks employ one of two types of transmission modes: ASCII Mode and RTU
Mode. The mode of transmission is usually selected along with other serial port communication
parameters (baud rate, parity, etc.) as part of the device configuration. VPVision supports Modbus
RTU only.
Pin layout:
A aka '- ' aka TxD-/RxD- aka inverting pin
B aka '+' aka TxD+/RxD+ aka non-inverting pin
SC aka G aka reference pin
ALWAYS use the SC/ Ground reference! Without reference, Modbus networks might work for a while
but eventually, you could run into communication issues due to capacitive effects or electromagnetic
interference.
