Multi-network MODBUS/RS-485 Protocol
Publication Date: 2014-Aug-25
The IP.com Prior Art Database
Disclosed is a method to add source routing techniques to MODBUS*/RS-485, providing scalability and significantly improving the device capacity of architecture. The novel header stacking minimizes changes at the end MODBUS device required to implement scalability and eliminates the need to maintain state at the Gateways.
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MODBUS* is a master-slave link (International Standardization Organization (ISO) layer 2 or L2) and application (ISO layer 7) messaging bus protocol. Figure 1 shows one adaptation of MODBUS with L2 framing data in blue and application data in green . In general, the application layer can be used to implement a read /write memory abstraction, which can be used to operate most any device type and collect data from most any sensor type.
Figure 1: MODBUS (L2 framing data in blue and application data in green )
Combined with the RS-485 physical and electrical signaling layer, MODBUS has been a simple and elegant architecture allowing millions of automated devices to communicate . The moderately high-speed transmission rate and 1,000 meter cable length has made MODBUS superior for physically distributed environments. However, the MODBUS/RS-485 combination has some serious limitations. The 256 device limitation is a function of RS-485 signaling. Thus, even though the devices can be distributed over a large area, there can typically only be 256 present.
The present contribution is a method to add (ISO layer 3) source routing techniques to MODBUS/RS-485, providing scalability and significantly improving the device capacity of architecture. The novel header stacking minimizes changes at the end MODBUS device required to implement scalability and eliminates the need to maintain state at the Gateways.
By creating Gateway devices, a hierarchical network of MODBUS segments can be configured. In Figure 2, each segment is a state-of-the-art 256 device MODBUS/RS-485 network. In the example, the Master device at the top layer constructs an encapsulated message that includes all Gateways (light blue boxes) in the path of the Slave device. The Master then sends the message to the first Gateway . The first Gateway partially unpacks the message and sends it to the second Gateway . This unpacking process continues until the message reaches its destination . There are established methods for managing Routing tables used to determine the Gateway path to any device in a hierarchical network. This disclosure as...