ST AN1702 APPLICATION NOTE

AN1702

Application note

ST485EB for e-meter applications

Introduction

This application note explains the procedure for designing the RS-485 communication lines in energy metering applications which helps e-meter engineers build their projects.

Energy meters are used to monitor personal usage of electricity. In the past only one kind of e-meter was used which was an electromechanical device and required human intervention for the periodic on site data collection. The new frontier in energy metering is a new generation of electronic e-meters providing new features such as network data collection, auto-diagnosis and time-scheduled accounting differentiation in order to define different energy costs on differently defined time segments.

These meters provide monitoring, analysis and data storage on energy usage.

The network connection between meters allows for centralized data collection with the significant advantage that manual meter reading is no longer necessary. Moreover, it makes locating meters easier, since external accessibility (in order to get a manual reading) is unnecessary.

The network architecture can be of different types: bus, daisy chain, tree structure and so on. One of the most efficient is bus architecture. Each meter is connected to the main bus through a stub, which is kept as short as possible. The communication should be bidirectional. Each meter, in fact, needs to send data, as well as receive data from the central control, such as permission to send tariff upgrades, diagnostic routines, setups, etc.

Modern communication technologies offer several choices of communication with meters. One of these methods is the use of a twisted pair cable, using RS-485 differential transmission. It offers a wide choice of transceivers available on the market, high data rates over long line lengths, high noise rejection, and good electrical robustness that makes it suitable in rugged environments.

September 2007

Rev 2

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www.st.com

Contents	AN1702

Contents

1	RS-485 interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 4
2	RS-485 advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
3	Signal levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
4	Bus lengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
5	Network isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
6	Fail-safe biasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
7	Transient voltage suppression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
8	ESD protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
9	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10

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AN1702	List of figures

List of figures

Figure 1. RS-485 half-duplex network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 2. RS-485 full-duplex network. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 3. Dangerous ground shifts in a common situation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 4. RS-485 terminal node with optocouplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 5. External fail-safe and line DC termination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 6. IEC 61000-4-5 transient voltage test V and I waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 7. Usage of the transient voltage suppressors in the RS-485 network. . . . . . . . . . . . . . . . . . . 9

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RS-485 interfaces	AN1702

1 RS-485 interfaces

With RS-485 two kinds of communication lines can be built: half or full duplex. A full duplex line requires four wires, and each node can simultaneously send data on two wires, while receiving from the other pair of wires. A half duplex line uses only one pair of wires for both sending and receiving, and in this case only one node at any one time can send data on the line (see Figure 1 and Figure 2). This means that two or more nodes, sending contemporaneously data to the bus, cause errors in data transmission. It is the application engineer’s responsibility to choose the proper network management method in order to avoid this problem. This choice may be constrained by the need to use a pre-defined protocol on the network, or by the limitation of the calculated power of the network controller.

Figure 1. RS-485 half-duplex network

Figure 2. RS-485 full-duplex network

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