About
Modbus is a serial communications protocol. Modbus has become a de facto standard communication protocol and is now a commonly available means of connecting industrial electronic devices. The main reasons for the use of Modbus in the industrial environment are:
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Modbus enables communication among many devices connected to the same network, for example, a system that measures temperature and humidity and communicates the results to a controller/monitoring unit.
Protocol versions
At the moment Vutlan supports the following Modbus versions:
Modbus RTU — This is used in serial communication and makes use of a compact, binary representation of the data for protocol communication. The RTU format follows the commands/data with a cyclic redundancy check checksum as an error check mechanism to ensure the reliability of data. Modbus RTU is the most common implementation available for Modbus. A Modbus RTU message must be transmitted continuously without inter-character hesitations. Modbus messages are framed (separated) by idle (silent) periods
Communications and devices
Each device intended to communicate using Modbus is given a unique address. In serial and MB+ networks, only the node assigned as the Master may initiate a command. On Ethernet, any device can send out a Modbus command, although usually only one master device does so (Modbus TCP/IP will be supported by Vutlan in the coming future). A Modbus command contains the Modbus address of the device it is intended for (1 to 247). Only the intended device will act on the command, even though other devices might receive it (an exception is specific broadcastable commands sent to node 0, which are acted on but not acknowledged). All Modbus commands contain checksum information to allow the recipient to detect transmission errors. The basic Modbus commands can instruct an RTU to change the value in one of its registers, control or read an I/O port, and command the device to send back one or more values contained in its registers.
RS-485
RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement Industrial Control Systems, including the most common versions of Modbus.
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Data A , which is low for logic 1 and high for logic 0.
Data B, which is high for logic 1 and low for logic 0.
Signal ground aka GND aka COM aka reference pin.
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Connecting Modbus devices
The total cable length should not exceed 1200 meters. The last devices in this chain must be terminated with a 120 Ohm resistor (The resistance of the twisted pair cable). Make sure both resistors are connected to the terminals correctly. The MODBUS specifications call for termination resistors (120 to 150 ohms, 0.25W) on both ends.
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VT900 and VT960 units use "RS485 to USB adapter" to connect Modbus RTU devices. "RS485 to USB adapter" is ordered separately.
Read more at:
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Questions and Answers
What are the data wires?
RS-485 is a differential signaling method. What this means is that the same data is transmitted on both wires, but in exact opposite polarity. When there is logic 1 on one wire, there is logic 0 on the other, and this happens at a fast rate (the baud rate). By running these two opposite signals on a twisted pair of wire, high data rates at long distances can be achieved because any noise induced into the lines is induced equally in both of the opposite polarity signals and therefore cancels out.
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