September 2007

Rev 2

1/11

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 bi-

directional. 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.

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