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Using Modbus to Communicate with Multiple OZpcs-RS40s

The OZpcs-RS40 is a 40kW Power Conversion System (PCS) intended for battery-based energy storage applications. The PCS is designed to be mounted in a standard 19” rack, and easily paralleled to scale power capability. All hardware interfaces are located on the front panel, utilizing pass-through type terminal blocks for simple daisy chained cable or bus bar power connections. Similarly, the Modbus and digital I/O signals are provided on redundant, high density, 15-pin D-Sub connectors, which also allow for simple daisy chain cabling. When using Modbus to control multiple, paralleled PCS there are several things to consider, including termination, addressing, and broadcast messaging.

The OZpcs-RS40 provides Modbus RTU serial communications over an RS-485, multi-drop, hardware interface. Modbus RTU is a Master/Slave protocol where only one node, assigned as the master, may initiate a command. All other devices are slaves and respond to requests and commands from the master. The OZpcs-RS40 is designed to operate as a slave device.

The Modbus RS-485 data bus must be terminated for reliable communications. However, only terminate devices physically located at the end of the wires. Don’t terminate every device on the bus, as this will degrade the signals and overload the transceivers. The OZpcs-RS40 provides an internal termination resistor that may be enabled by shorting pins 4 & 5, and 14 & 15 on the last PCS in the daisy chain.

Whenever two or more units are paralleled on the same RS-485 serial bus, each PCS must be assigned a unique Modbus Device Address so it can be individually addressed by the system controller. The configuration register REG 40069 – Modbus Device Address is used to set the address of each PCS. The factory default setting for this register is 1, and it supports an address range of 1 to 247. Each PCS must be assigned a unique address by writing to this register before attempting to connect and operate it in parallel with others. The register is non-volatile, so the register only needs to be written once; its value is retained when power is removed.

The Modbus protocol defines a broadcast write mechanism that uses the Modbus device address of zero to indicate that all slaves should respond to the write message regardless of their unique slave address. This is useful if you want to turn multiple devices on or off, or change their operating setpoints simultaneously.

For example, assume you have a system with three OZpcs-RS40’s assigned Modbus addresses of 2, 3 and 4. You can set the power output of each one individually by writing to each PCS, i.e., one Modbus write to address 2, another to 3, and a third to 4. Alternatively, they can all be set simultaneously by a single Modbus write to address 0.

The OZpcs-RS40 is a MESA/SunSpec compliant, rack mounted PCS, specifically designed for grid-tied and micro-grid ESS applications.  Download the product brochure (DR-0110) for additional information.