Concept
Legrand Power Meters
This application note provides recommendations and guidelines for the configuration of RS-485 Modbus communications between the automation server and Legrand power meters. The recommendations are based on an investigation of Legrand meter models 04677, 04680, 04684, and 14669. The investigation was used to discover isolation and unit load characteristics along with requirements of the RS-485 transceiver for external failsafe biasing on the RS-485 network and to test communications operation with the meters under recommended configurations. This information is intended to supplement the product information provided with the Legrand meters. The guidelines focus on the arrangement of the electrical interface to the automation server's RS-485 port in regards to biasing, termination, cable selection, cable lengths and cable routing. The guidelines on maximum unit load (node count) and common mode voltage tolerance are associated with AS-P and AS-B servers.
General Legrand Power Meter Properties
An important difference between the Legrand meters is that models 04677, 04680, and 04684 use transceivers with integrated failsafe receivers, whereas the model 14669 uses a transceiver with no integrated failsafe function.
For more information, see General Legrand Power Meter Properties .
Legrand Configurations
The recommendations include four different configuration options to choose from with differences in performance and/or resources required.
For more information, see Legrand Configurations .
Unit Load Definition, Maximum Network Load and Affects of Excess Unit Load
According to the TIA-485A standard, a single unit load is equivalent to a 12 kohm impedance attached to the + and – data lines (connected to ground or supply). A 1/8UL transceiver would have an impedance of 96 kohm. The TIA-485A defined total network load limit of 32UL is based on a common mode load resistance of 375 ohm connecting both the + and – data lines to ground (or CMV source). The standard requires the RS-485 drivers be capable of driving a network load of 32UL along with a Common-Mode Voltage (CMV) difference of -7 V to +12 V and produce a guaranteed minimum of 1.5 V transmit signal level. Such a full UL load with severe CMV conditions exhausts the maximum drive current of 60 mA provided by all standard RS-485 drivers. The specified minimum of 375 ohm resistance for the common mode load is the resulting resistance seen when 32 transceivers with 12 kohm input impedance are placed in parallel (12,000 / 375 = 32).
For more information, see Unit Load Definition, Maximum Network Load and Affects of Excess Unit Load (Legrand Power Meters) .
Expanded Unit Load with Network of Isolated Devices Only
If the network is comprised exclusively of devices with isolated RS-485 interfaces with the only exception being the automation server, it is recommended that the maximum unit load limit can be stretched higher. With a network comprised exclusively of isolated interfaces connected to the automation server, the standard unit load budget of 32UL can be increased by 50% to have an expanded budget of 48UL. Using a maximum network load of 48UL and subtracting the 24UL for the bias network resistors and the load of the automation server leaves 24UL available for the nodes to be connected on the bus. With the example device load of 1.32UL each, the expanded budget of 48UL supports 18 nodes (meters) on the bus.
For more information, see Expanded Unit Load with Network of Isolated Devices Only (Legrand Power Meters) .
Cable Routing
The RS-485 network cable should be routed in a continuous daisy chain bus configuration. There should not be any stub connections, stars or ring configurations. The bussed cable should pass through each node to be connected with no splits or branches in the cable network.
For more information, see Cable Routing .
Cable Selection
This is one of the most important selections having significant impact on the performance and reliability of the RS-485 network being installed. An incorrect cable selection can be difficult and expensive to reverse. The decision should not be made on previous examples of seeing some alternate non-compliant cable work.
For more information, see Cable Selection (Legrand Power Meters) .
RS-485 Communications
General Legrand Power Meter Properties
Legrand Configurations
Unit Load Definition, Maximum Network Load and Affects of Excess Unit Load (Legrand Power Meters)
Expanded Unit Load with Network of Isolated Devices Only (Legrand Power Meters)
Cable Routing
Cable Selection (Legrand Power Meters)