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The Modbus protocol was released in the 1970s as a way to ease communication between different devices, such as generators, PLCs, and even reverse osmosis pumps.
Modbus is an open-source communications protocol, which led to the widespread deployment of Modbus systems which are most frequently used for industrial applications. Multiple Remote Telemetry Units (RTUs) and/or Intelligent Electronic Devices (IEDs) that supports the Modbus protocol can be connected to the same physical network to create a Modbus network.
A Remote Telemetry Unit (RTU) is a device created for monitoring and reporting events that occurs at a remote site. The typical events that will be monitored are temperature, humidity, and voltage levels. Almost anything can be monitored with an RTU. Information regarding these events is collected by the RTU and then sent out to a master station like the T/Mon.
And an Intelligent Electronic Device (IED) is a microprocessor-based controller that has advanced local control intelligence.
A Modbus network, at its most basic, is a network of master and slave devices that communicate using the Modbus protocol.
The master/slave address design of a Modbus network enables a Modbus master to communicate with a specific device by using its unique Modbus address. This unique identifier, the address, has a range of 1 to 247 and can be located at the beginning of the message that will be sent to the Modbus master.
A simple Modbus network can have up to 247 connected at one time the same network. Each of the 247 connected Modbus devices is assigned their own unique identifier which they will use to communicate with other Modbus devices.
Note: Modbus TCP/IP networks can generally support more devices than a traditional Modbus serial network.
In a master and slave configuration, the communication stream is controlled by the Modbus master. The Modbus master can request data from or write data to Modbus slave devices across the network using a specific register and/or coil addresses. The Modbus slaves on the other-hand can only respond to the master's requests or commands as it is received.
These Modbus networks typically utilize a serial connection - a serial connection, or serial communication, is the process of sending data one bit at a time over a connection or communication channel in a sequence.
While Modbus TCP/IP networks, which are made up of devices that support the Modbus TCP/IP protocol, will usually use 10BaseT or faster for their connection. 10BaseT is a twisted-pair cable that is used for LAN and can achieve speeds of up 10Mbps.
In other words, TCP/IP is used as the transport protocol of the Internet and is made up of two different protocol layers.
The first layer, the transport layer, called the Transmission Control Protocol (TCP) is used to break up a message or file into smaller pieces of data called packets that are then sent over the Internet. It is also responsible for rearranging the packets when it receives them.
The second layer, the Internet layer, called the Internet Protocol (IP) is used for addressing the packets so that the packets will get to the correct destination.
The Modbus Bridge allows Modbus devices to work on a lot of types of networks.
A Modbus bridge is a device that connects Modbus serial products to Modbus TCP Masters. It can be an important part of a Modbus network.
Modbus serial is the original variant of Modbus that travels over a serial connection. This serial connection commonly uses an RS-232 or RS-485 serial interface. Modbus TCP is an extra variant of the Modbus protocol. Internet users can access Modbus TCP at port 502, which is reserved for Modbus users on the TCP/IP stack.
Today, Modbus TCP has become a popular variant of the Modbus protocol. A Modbus bridge will allow users to transition to this variant, while still making use of their Modbus serial gear.
A Modbus bridge may be configured as a Slave or a Master bridge, simply by connecting to either device serially. A Modbus bridge can also be used for other Modbus variants, such as Modbus ASCII and Modbus RTU.
With a Modbus bridge, users can connect thousands of slave devices using a single Ethernet card and the Modbus TCP protocol. Devices joined to a Modbus bridge have single IP addresses, and addressing slaves through the Modbus bridge is now defined by the IP address and the slave address or unit id.
When a Modbus bridge receives a Modbus TCP request, it converts the messages into Modbus RTU or other variants of the Modbus protocol, creating a response using Modbus TCP. In these cases, the Modbus master most likely does not even see that it is not actually communicating with a Modbus TCP device.
A Modbus bridge can connect to many Modbus TCP masters at one time. Serial slaves will actually see the Modbus bridge as the master device. The actual TCP Modbus masters will treat the Modbus bridge like a group of slaves, communicating as if every single master had sole access to the slave devices. With more masters accessing the Modbus bridge, response times can become slightly sluggish.
With an ordinary master, you will be unable to collect alarms from your non-Modbus devices. This will require you to deploy multiple masters to support all of the different protocols within your network, as well as hire additional operators to monitor the communications from these assorted masters.
A Modbus network should be developed using the most advanced Modbus monitoring technology. With advanced Modbus monitoring systems, you can provide automatic text message and email notifications of all your alarms. With location and repair information sent directly to your technicians, you can handle network problems more quickly and efficiently. The most advanced systems will also allow you to bring in alarms from other protocols and can display all of your important notifications in a single window that can be monitored by a single network operator.
Modbus projects tend to involve a lot of other protocols and equipment, so you need a master system such as the T/Mon NOC can be what you need for your Modbus network. It is a multifunction and multiprotocol system that provides a multitude of features all in a single platform.
Modbus, SNMP, and ASCII are just a few of the many protocols supported by the T/Mon LNX. The T/Mon LNX comes with T/Mon GFX which is a graphical user interface for displaying alarms visually on a layered geographical map. Built to be the solution for a wide range of problems, it could be exactly what you need.
No matter what protocol you choose to use, you must never forget certain things.
Always focus on open protocols that don't trap you. By reading this article, you've already done that.
Open standards empower you to choose a new manufacturer later. Closed protocols written by one manufacturer don't do that.
Also, look for devices that speak in multiple protocols. You should always standardize as much as possible, but something WILL come up.
You'll want a piece of equipment, but it only understands DNP3 or CANBUS. Without the right master station, good luck.
So, even if you don't need it today, look for a multi-protocol master station.
If you work with any kind of legacy equipment, including serial Modbus, you need very specialized gear to make it work in today's networks. That's where your choice of manufacturer comes in. Some just sell commodities. If you need a very unique solution, you're out of luck.
What you need to get a custom Modbus solution is a manufacturer who can take a proven design and adjust the design to fit your needs. Look for a company that does its own engineering and manufacturing. They need in-house teams in those areas to be responsive to your special requests. Otherwise, they'll have a really hard time meeting your specs without an insane minimum order.
A company that farms out everything except for their "core competency" simply can't react with the speed that an integrated company can. You need a manufacturer whose core competency is being flexible and giving you exactly the Modbus solution you need.
We've developed a new way to prioritize the polling of thousands of Modbus registers. This helped one of our clients monitor hundreds of registers spread out over several devices while focusing primarily on the few that matter the most.
You may not need to process large amounts of Modbus information. But what do you need? Remember that we can customize a solution that will directly meet your unique needs.
We not only can develop custom firmware to meet your very specific technical requirements but also your new firmware functions will run on our proven hardware and benefit from our remote monitoring expertise.
Treat us like your engineering department, simply call us today and tell us what you want.
To protect yourself and your company, you need to get educated. The more you know about Modbus and SCADA generally, the better. The SCADA white paper is a great place to start learning.
To see extra information related to a "Modbus Bridge", please visit the Modbus Network page.