NOJA Power

Technical Article

Published 01/2021

Modbus in Reclosers

Modbus is one of the oldest and most beloved of communications protocols for Supervisory Control and Data Acquisition (SCADA). Developed in 1979 by Modicon, Modbus was originally designed for Programmable Logic Controllers. Today, Modbus owes its popularity to being both simple and royalty free, despite disadvantages against more modern SCADA protocols. In spite of its drawbacks, Modbus remains actively used in many legacy process and electrical utility control environments.

NOJA Power’s latest firmware release for the NOJA Power RC-10/RC-15 controllers includes an implementation of Modbus. This allows the RC-10/15 to be integrated into existing Modbus SCADA architectures. As with most engineering decisions, selecting Modbus as a SCADA protocol carries both advantages and disadvantages. We explore these in the table below.

Advantages and Disadvantages of Modbus Protocol

Royalty FreeModbus provides no security features. There is no protection against unauthorized commands, or data interception
Lower CostModbus does not support large Binary objects
Lower maintenanceModbus provides no description of data objects.

Serial implementations of Modbus carry additional limitations, such as the number of devices on a data link (maximum 247), and a lack of report by exception.

“Due to customer demand to integrate our reclosers into legacy communications and Scada systems that use Modbus protocol, we have implemented the Modbus protocol into our SGA software suite,” reports NOJA Power Group Managing Director Neil O’Sullivan. “This allows easy configuration and download of the Modbus device file into our controllers from the SGA software suite.”

Modbus Use Case

Modbus can be considered a sound engineering decision when there is no requirement for compliance to network security standards and costs must be optimized. Generally, this requires wired and physically secured communications media, but it is less appropriate where wireless communications are involved. Wireless broadcasting of Modbus protocol could plausibly be intercepted by any third party within communication range, exposing a significant attack surface.

Legacy systems which are scheduled for upgrades to achieve compliance with cybersecurity standards can be progressively upgraded by replacing failed RTU equipment with devices that support both Modbus and more modern SCADA protocols (such as IEC 61850, IEC 60870 or DNP3-SA). This method removes the need for entire system obsolescence, instead providing an upgrade path over time and maximizing asset utilization. Eventually, when all individual outstation devices are replaced, a new SCADA server can be implemented, and the modern outstations can be reprogrammed to use their modern SCADA protocols. An example of this upgrade process is shown in the diagrams below:

Stage 1: Simplified legacy Modbus control environment
Stage 2: Progressive replacement of legacy outstations with devices compatible with both Modbus and Modern Protocols
Stage 3: Change Server to Modern Protocol with Security Compliance

Modbus remains a popular protocol for SCADA due to simplicity and cost effectiveness. For physically secure environments, it represents a sound engineering choice for SCADA control at a low maintenance overhead. NOJA Power’s OSM Recloser system can now be directly integrated into these legacy environments, while also providing an upgrade path for the system to more modern protocols when needs arise.

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