The Efficiency Benefits of using Modbus as a Communication Protocol in Process Automation

Within the realm of controls engineering, a communication protocol can be simplified as being a method in which data is communicated within a system.

In process automation, some of the most common protocols used are as follows:

  • Ethernet/IP
  • ControlNet
  • DeviceNet
  • EtherCat
  • PROFINET
  • Profibus
  • EtherCAT
  • CC-Link
  • HART

Many of these protocols were developed to support specific PLC manufacturer’s products as a method of serial communication. However, this article is going to focus on one specific protocol that can be used almost universally; and that would be ModBus.

Nearly any device that utilizes Modbus communication, can be used in any control system. Whether it be controlled with a PLC using Allen Bradley, Siemens, Omron, GE, Schneider, Bachmann, ETC.

To fully understand the advantages of using Modbus devices, we must first with start from the building blocks of creating a program for a control system using any given PLC platform.

Assuming the user understands how to create and configure their I/O tree with all cards and ethernet devices active, the next step is creating logic which often involves using the process data for a given device, whether it be an analog input or analog output Digital I/O rarely involves an in-depth amount of knowledge of process data within a device, with the exception of HART devices or devices that utilize a lot of user created digital signal processing, but for the sake of simplicity we will ignore that for this article and revisit it later.

Let’s first look at how process data within a device works.

Figure 1.

Now let’s give a hypothetical example and say we have a temperature sensor on a line that operates from on a 24V digital signal and shows an output of 4-20mA which is then converted to 0-100 Fahrenheit for operations. Meaning a current of 4mA is 0 degrees and 20mA is maximum temperature, while no current at all means some sort of signal failure or device failure has occurred.

This pressure device could have 1 byte of analog output data which is sent to the PLC which converts the electrical current into a usable figure (i.e. degrees in Fahrenheit) for the user. As 1 byte can contain any output value from 0-255, this needs to be converted in the program.

Now typically this process is done by using either an Add-on-Instruction or a function block diagram by sending the correct bits exactly where they need to go on the analog input card of the PLC rack in the electrical cabinet.

Luckily, this is where the convenience of Modbus devices come in.

Modbus allows full configuration with both master and slave devices using your web browser with a simple ethernet connection. All IP addresses, set-points, and ranges can simply be configured in your browser without having to make any edits to the program. Any signals that need to interact with an existing system can be linked through a hardwired connection to the I/O rack even without any ethernet communication between the master device and the PLC or managed/unmanaged ethernet switch.

A great example of this would be the x400 series of devices by ControlByWeb (not a sponsor or in any way affiliated with Integral Magazine).

ControlByWeb provides an amazingly easy to use interface in which you can configure your device with its setpoints and ranges without the use of AOI’s or function blocks. As well as allowing the ability to send email or text alerts to engineers or technicians with the correct SMTP settings configured.

Most importantly, Modbus allows for constant back-end data monitoring with its in-browser SCADA system. This along with the option to receive emails as soon as alarms occur, allow for real-time changes and maximum efficiency in operations of any plant.

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