December 2015

Efficient Modbus-to-PROFIBUS Conversion in Modern Automation Systems

Modern automation control systems can significantly decrease the total cost of ownership by using intelligent field devices to further enhance savings. Since most of these devices implement Modbus RTU, a simple but cost-effective communications protocol, a common challenge is the integration of Modbus RTU devices with PLC control networks that use PROFIBUS.

Why the Typical Solution is Inefficient

Not every intelligent device vendor is able to invest in cutting-edge industrial communications for field devices. Such capabilities are still not seen as contributing to the core value of these devices. Most of these devices continue to implement the Modbus RTU protocol because of its low-cost and simplicity. Customers typically would ask the PLC vendor to provide a communication module to extend the Modbus interface by means of an add-on to the PLC. Thus, a common requirement seen when designing modern automation control systems is to integrate low-cost Modbus RTU devices into PLC control networks.

An example of how a cost-effective and efficient solution was achieved can be seen in a cold storage warehouse located in one of the world’s biggest ports, which has a size comparable to 10 football fields. Large volumes of goods, mostly food and beverages, need to be kept at low and stable temperatures to maintain freshness. Engineers designed an automated control system with a decentralized architecture that uses remote IOs to collect information such as the environmental temperature and pressure of coolants in the refrigeration cycle, and then transmits the information to a PLC. The PLC uses information collected from remote sensors to control the action of fans that keep the temperature within a specified range. These “intelligent fans” provide feedback on the status, speed, operating times, motor temperature, and other parameters, and in addition can be controlled remotely by a PLC. Since the fans use the Modbus RTU protocol and the PLC uses PROFIBUS, there was clearly a need to find a way to connect the two incompatible systems so that they could talk to each other.

Engineers often face the following issues when implementing a typical PLC-based communication module:

Issue #1: Higher implementation costs
The cost of a PLC communication module is almost the same as that of a basic PLC CPU. Depending on the size of your project, several such communication modules must be installed as extensions to the PLC module in the control room. Serial cables are used to connect the field devices to the communication modules.

Issue #2: Not very effective in a system with distributed architecture
The PLC-based solution is not suitable for an automation control system that consists of remote devices and IOs, especially when these devices are installed hundreds of meters away from the control room.

Issue #3: Long cables
One serial cable is needed per PLC communication module, and several communication modules are used in distributed architecture. This increases the complexity of installation. To make matters worse, most of these devices are located hundreds of meters away from the control room, which adds to the cost of the system.

Issue #4: Additional programming
Extensive programming is required to enable various commands, which is a complicated task for those who are not experts in communications protocols.

Overcoming the Problems of a Typical Modbus-to-PROFIBUS Solution with Moxa’s MGate 4101-MB-PBS

The MGate 4101-MB-PBS is an easy-to-use, cost-effective solution that can withstand extreme temperatures ranging from -40 to 75°C. The MGate is certified by IECEx and provides various troubleshooting tools to reduce downtime.

  1. Protocol conversion: The MGate can actively establish connections to Modbus RTU devices, collect and store information, and then respond to PROFIBUS PLC queries with data stored in its memory.
  2. Easy configuration: An easy-to-use configuration utility eliminates the need for engineers to write complicated code that would generally require a lot of fine-tuning.
  3. Effective troubleshooting:
    Realy outputs provide a smart alarm function quick identifies system faults, thereby reducing costly downtime.
    • The Modbus Traffic Monitor displays the actual Modbus data flow, making it easier to identify communications errors.

The customer was clearly ready for a different solution to solve the protocol conversion issue between PROFIBUS and Modbus RTU without the burden of the problems listed above.

For more information about this product, please visit the product page.

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