Given the Modbus data address, the PLC address is determined as:įor example, if the Modbus address for a discrete output coil is 22, then the most significant part is 2 (22 / 8) and the least significant part is 6 (22 mod 8). The least-significant part of the PLC address has a range 0 to 7, therefore, a little math is needed to translate between PLC addresses and Modbus data addresses. Platforms based on ATmega 328P, ATmega 168, ATmega32U4, and ATmega16U4 like the Arduino Uno and similar have an even more limited address space due to their very low RAM footprint: Modbus Data Typeĭiscrete output coil and discrete input contact binding are based on the the two-part PLC address, without unused Modbus data addresses. The following table shows the Modbus address space for the OpenPLC Linux/Windows runtime: Modbus Data Typeĭue to memory limitations, embedded platforms like Arduino boards have a much more limited Modbus address space, according to the table below: Modbus Data Type Addresses are mapped sequentially whenever possible. lower PLC addresses are mapped to lower Modbus addresses. The Modbus addresses bind to PLC addresses based on the hierarchical address value, i.e. AB7042 MODBUS SERVER SERIALWrite multiple analog output registers (0x10) Anybus AB7072 EtherNet/IP, Modbus-TCP Serial converter RS-232, RS-422, RS-485, Sub-D9 galvanic isolation, Ethernet 24.Write analog output holding register (0x06).Read analog output holding registers (0x03).Write multiple discrete output coils (0x0F).OpenPLC supports the following Modbus function codes: This configuration can be changed on the “Settings” tab. The OpenPLC Runtime for Linux and Windows hosts has a Modbus server running by default on TCP port 502. The OpenPLC micro runtime for Arduino-compatible boards also supports Modbus frames over serial and USB. The remote master initiates read and write requests to the OpenPLC slave sending Modbus frames over the network (Modbus/TCP). Read Data from a Modbus Server Use the modbus object to read coil, input, and register data from the Modbus server. Configure Properties for Modbus Communication Use properties to set up Modbus communication during or after object creation. A Modbus slave is normally controlled by a master that performs measurement and control. Create a Modbus object with the modbus function, to connect to a Modbus interface. For example, a large server may be used to master a programmable logic controller (PLC) or programmable automation controller (PAC), while that PLC/PAC may in turn master a sensor, valve, motor, or any other embedded device. OpenPLC can be configured as a Modbus slave (server). Modbus is typically used for Supervisory Control and Data Acquisition (SCADA)-style network communication between devices.
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