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Electronic energy meters steadily are replacing the historical, electromechanical meters in industrial plants, commercial buildings and domestic communities. These new type of meters can record energy usage at different times of day and even different forms of power (i.e., real versus reactive) to enable multi-rate billing. They provide improved measurement accuracy and significant lower power consumption than mechanical meters.
An electronic energy meter (e-meter) inherently is programmable, allowing a basic hardware design to be easily reconfigured by software for different applications. A major benefit is the provision of automatic meter reading (AMR), the ability to collect data via networked communication.
Because energy meters operate in harsh and noisy environments, robust RS-485 differential signaling has become the preferred method of remote data transmission.
This article helps design engineers to build a robust e-meter network by providing design guidelines for RS-485 communication circuits.
Energy Meter Functional Overview
Figure 1 shows the block diagram of an e-meter. The incoming line voltage, L1 to L3 and Neutral, are attenuated via potential dividers in the voltage sensor block, while the line currents are measured via shunt resistors in the current sensor unit. Their analog outputs are converted into digital data through the A-to-D converter stage in the metrology processor, and also are fed back to the terminal block making the e-meter transparent to the electrical installation.
Figure 1. Simplified block diagram of an electronic energy meter. Click here for full image.
The metrology processor performs four-quadrant multiplication to determine the amount of active power consumed, as well as the amount of reactive power loading the mains. The calculated results are forwarded to the system controller, which, besides display and memory management, controls the data transmission between a meter and the central data collection point via an RS-485 interface.
Bus Implementations
As RS-485 requires the e-meter nodes to be networked in a bus structure (Figure 2), the interface bus can be designed for half-duplex or full-duplex transmission (Figure 3).
Figure 2. Bus structure of energy meter network using RS-485. Click here for full image.
The full-duplex implementation requires two signal pairs (4 wires), as well as full-duplex transceivers with separate bus access lines for transmitter and receiver. Full-duplex allows a node to simultaneously transmit processed data on one pair while receiving configuration data on the other pair.
In half-duplex only, one signal pair is used requiring the driving and receiving of data to occur at different times.
Both implementations necessitate the controlled operation of all nodes to ensure that only one driver is active on the bus at any time. Having more than one driver accessing the bus at the same time leads to bus contention, which must be avoided through software control.
Figure 3. Half-duplex and full-duplex bus structures in RS-485. Click here for full image.
Balanced Signaling
E-meter applications benefit from differential signaling over twisted pair cable because external noise from industrial machinery and home appliances couples equally into both signal lines as common-mode noise, which is rejected by the differential receiver input.
Equipment design using RS-485 communication must maintain the balance signaling approach of the connecting network. Cable selection, connector pin assignment, and circuit board layout should keep both signal lines closed and equidistant from another to maintain electrical characteristics.
Suggested network cables are of the sheathed, unshielded, twisted pair type (Figure 4) with a characteristic impedance of 120 and 22 " 24 AWG.
Figure 4. Example of RS-485 Communication cable.
Signal Levels
RS-485 standard conform drivers provide a differential output of minimum 1.5V across a 54 load, while standard conform receivers detect a differential input down to 200 mV (Figure 5). The two values provide sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. This robustness is the main reason why RS-485 can be used for long distance networking between e-meters, especially in noisy environments.
Figure 5. RS-485 specified minimum bus signal levels. Click here for full image.
Bus Loading
Because a driver's output depends on the current, it must supply into a load. Adding receivers to the bus increases the total load current required. To estimate the maximum number of bus loads possible, RS-485 specifies a hypothetical term of a unit load (UL), which represents a load impedance of approximately 12 k. Standard compliant drivers must be able to drive 32 of these unit loads in addition to the 120 termination resistors at each cable end. This drive capability of 32 UL is also required by the DL/T645 standard. Today's transceivers often provide reduced unit loading, such as one-eighth UL, thus allowing the connection of up to 256 transceivers on the bus.
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