Modbus is a standard protocol for serial communication. Popular across industries, it is often used to connect various industrial electronic devices and instruments to a system that gathers data or a main controller. A common example would be a device that measures temperature and humidity level and sends collected data to the control center via a computer.

1. Modbus Programming Examples
2. Modbus Over Serial Line V1.02
3. Modbus Rtu Communication

Modbus is a serial communication protocol. Originally developed by Modicon in 1979. Use with Modicon PLC. Modbus is a very simple protocol. Modbus protocol also defined as a master-slave protocol. Means device operating as master will poll one or more device operating as slave.master user read data from slave device register. Master user Write data from slave device register.

1. This document describes the MODBUS over Serial Line protocol. MODBUS Serial Line protocol is a Master-Slave protocol. This protocol takes place at level 2 of the OSI model. A master-slave type system has one node (the master node) that issues explicit commands to one.
2. An Introduction to Modbus RTU Addressing, Function Codes, and Modbus RTU Networking Overview Let’s Break Down Modbus RTU. Modbus RTU is an open serial protocol derived from the Master/Slave architecture originally developed by Modicon (now Schneider Electric). It is a widely accepted serial level protocol due to its ease of use and reliability.
3. This document describes the MODBUS over Serial Line protocol. MODBUS Serial Line protocol is a Master-Slave protocol. This protocol takes place at level 2 of the OSI model. A master-slave type system has one node (the master node) that issues explicit commands to one of the 'slave' nodes and processes responses.
4. Modbus over Serial Line specification encourages the use of the multipoint EIA-485 standard — supporting up to 32 devices over a common bus.
5. Modbus is a “master/slave” communications protocol. There is one Master, which usually takes the form of a building automation system (BAS)communications gateway device, programmable logic controller (PLC), or datalogger software program running on a computer. There can be up to 127 slave devices on one multi-drop RS-485 serial bus.

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What is Modbus?

The serial communication protocol known as Modbus was developed and published in 1979 by Modicon® with the intent to deploy it in their programmable logic controllers (PLCs). The protocol is basically a technique of enabling data transmission between devices via a serial interface. The Modbus Master is the name for the device that requests data. Its data requests are handled by devices known as Modbus Slaves. There is a single Master in the standard Modbus network configuration, and up to 247 Slave devices uniquely identified by a Slave Address from 1 to 247. The Master has the ability to write to the Slave devices.

The official specifications of the Modbus protocol are defined here

Uses of Modbus

Manufacturers can incorporate Modus into their equipment royalty-free since it was developed as an open protocol. It is widely used when connecting industrial electronic devices and has evolved to become a common communications protocol used extensively in industry and manufacturing. The protocol is primarily used to enable the data transmission that is required between remote instruments and the central data server or controller. An example is a pressure monitoring device that updates the central computer through serial communication. Supervisory control and data acquisition (SCADA) systems often use the Modbus protocol to communicate between a remote terminal unit (RTU) and the supervisory computer. There are versions of the Modbus protocol available for (Modbus RTU and Modbus ASCII) and for Ethernet (Modbus TCP).

What is Modbus TCP/IP protocol?

Modbus TCP/IP is one kind of Modbus protocols designed for controlling and supervising equipment over TCP or IP. It is possible to use Modbus over Internet or Intranet if a device supports TCP/IP sockets and you know how to execute a TCP driver.
Else, there are alternative ways of doing it, we’ll talk about them below.

Sharing Modbus RTU/ ASCII over Ethernet

Software solution

This is special software - Serial to Ethernet Connector that allows you to work with Modbus to Ethernet. It enables you to provide shared access to a local Modbus device over network, so other network users can reach its content and functionality as if it were attached directly to their machines. In a similar way you too can access a remote Modbus over Ethernet regardless your physical proximity to the device. The number of devices you can work with over network can be up to 500, and it is possible to access all of them simultaneously.

To start using Serial to Ethernet Connector, download and install it on a computer that shares a local device (server) and computers that need to access it remotely over network (clients). Launch the app on the server and ‘Share’ a required device, then ‘Connect’ to it from remote computers – the device’s functionality becomes available to network users as it were a local device.

Serial to Ethernet Connector redirects Modbus RTU over TCP/ IP to remote computers. They, in return, display a virtual serial connection through which the software can communicate with the port.

Please note that this Modbus Ethernet converter runs like a Windows service. What does it mean? It means that whenever the system reboots, all connections get reconfigured automatically. The application runs in the background, and you don’t need to keep it open all the time. Another thing we’d like to bring to your attention is that the software can backup a configuration by moving it to another computer – that proves to be really handy at times.

The software is compatible with Windows and Linux operating systems, i.e. you can share a device on a Windows laptop and access it from a Linux desktop, or the other way round. The Linux edition is now available only as a command-line solution. Serial to Ethernet Connector can also be broadcast over UDP.

Serial Port Tunneling

Encapsulate Modbus messages

There is another way to use Modbus over IP or any other network called ‘serial port tunneling’. What you have to do here is to encapsulate a 'standard Modbus message' in to a TCP/IP packet or a UDP/IP packet and send it. This method can be used for sending nearly any serial protocol as if it were serial data flow.

However there is a catch here – if you are sending standard Modbus RTU over Ethernet, framing messages might require a while. Unless you have a good control over it, your messages might be split between packages and you will not receive them simultaneously. No content is lost, but because of delay the message won’t be received in a correct way.

To make sure your message is delivered in a single packet, you can use Modbus ASCII instead - this protocol allows you to specify Start of Message and End of Message characters.

Conclusions

You now have been introduced to the Modbus protocol. Users can work with the Modbus RTU over TCP IP protocol, but that does not solve all the issues involved with using Modbus over IP. In some cases, a Modbus RTU to TCP converter will need to be employed. An efficient software application is needed to assist with implementing Modbus RTU over Ethernet. When you need to use Modbus over the Internet, Serial to Ethernet Connector is a software solution that works as a Modbus to TCP IP converter. This application can be used to enact the Modbus RTU over TCP to connect PLCs as well as other electronic industrial equipment. Serial to Ethernet Connector is the most effective way to implement a software Modbus RTU to Ethernet converter.

Modbus Programming Examples

Serial to Ethernet Connector

There are some special techniques that can be employed when using an RS485 interface. Engineers currently supporting internet based automation may not be aware of these procedures. To close this knowledge gap, here is a short remedial course in RS485 vs MODBUS protocols.

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RS485 communication: Modbus vs RS485

What is RS485 communication? RS485 is a standard for serial data transmission that is similar to RS232 while using different electric signals. A benefit of the RS485 protocol is the ability to have several devices share the same bus. This means you do not need multiple RS485 interfaces to query multiple devices. A bus terminator must be used to accomplish this feat. You may need to move a switch to the proper position or screw a small resistor on a terminal.

You do need to use the correct interface for signals to flow as RS485 and RS232 are not directly compatible. You can use RS232 to RS485 gateways, but it is more common to go from RS485 to USB, GSM or Ethernet directly. This eliminates the need to an extra component and is the less expensive way to go.

Data exchanged through RS485 interfacing primarily uses the MODBUS protocol. On the other hand, RS232 devices use text (ASCII) protocols. This difference implies that you need to know a little about the MODBUS protocol when using RS485 communication devices.

Here is a short MODBUS tutorial

The MODBUS RS485 protocol defines communication between a host (master) and devices (slaves) that allows querying of device configuration and monitoring. MODBUS messages relay simple read and write operations on 16 bit words and binary registers often referred to as “coils”. Slave devices only respond to requests from the host which always initiates the conversation.

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When you have several devices connected on the RS485 bus in parallel, each device requires a unique MODBUS Slave ID. Every MODBUS request begins with the host contacting the Slave ID of the target device and the answer begins with the Slave ID of the sending slave device. For MODBUS communication to work correctly you need to ensure proper configuration of your devices’ Slave IDs. As with RS232 communication speed and parity also need to be in synch. Reg key windows 10 key.

ASCII protocols allow messages to be created by hand for the RS232 interfaces. This cannot be done with MODBUS as messages end with a checksum code that is computed from the complete message content. When exchanging MODBUS messages you need to use a specific proprietary program provided by your device vendor with a compatible interface. You can also use an RS485 interface and a programming library to encode and decode the MODBUS communication, or a Smart RS485 interface like the Yocto-RS485 that can handle the encoding and decoding itself.

There are two variations of the MODBUS protocol. In MODBUS ASCII mode messages are exchanged as lines of hexadecimal characters. Using the MODBUS RTU mode enables messages to be directly transmitted as binary frames. To communicate with a MODBUS device both host and device need to be using the same mode. MODBUS RTU mode is supported by all standard devices and is the mode most commonly used. MODBUS ASCII has no real advantage due to the difficulty in crafting messages by hand.

How to monitor and analyze Modbus Data?

Modbus Test Software is a sniffer tool that allows you to analyze RS232/RS422/RS485 interfaces transmitting MODBUS communications. SPM’s excellent functionality enables easy detection and resolution of problems encountered when engaged in MODBUS testing and debugging. A distinctive feature of this tool is how it can display and log all data flowing through your system’s COM port.

Using this MODBUS sniffer software’s advanced search and filter options, you can display only the relevant subset of data that you desire. SPM also contains a built-in terminal for executing text commands. This user-friendly application supports exporting data in various formats and has many customizable options.

This professional Modbus test software allows you to:

• Connect to and monitor serial ports already in use by another program. Real-time display of all data received by the port is available for immediate monitoring and debugging.
• Monitor multiple serial ports simultaneously. You can synchronously analyze the activity of several serial ports in a single session. Data flowing in both directions is presented on a first-in first-out basis in a single log for ease of viewing.
• Your data can be viewed in a number of formats including table, terminal, line, and dump views. Multiple views can be seen at the same time and you can use filters to fine-tune your monitoring.
• Perform emulation of data transferred to a serial device to monitor activity when exposed to specific commands.

Serial Port Monitor

Modbus Over Serial Line V1.02

Modbus Rtu Communication