AN1702

RS-485 advantages

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RS-485 advantages

Data transmission in e-meter applications occurs in noisy environments. Meters are

connected to the ac voltage, often located near industrial machinery or home appliances

and the transmission line is often very long (hundreds of meters) allowing atmospheric noise

to degrade signal quality.

For these reasons, RS-485 signaling is particularly relevant. Its differential drivers and

receivers, reject external noise (theoretically coupled equally into both lines) with high field

strength.

Project equipment using RS-485 signaling requires the application of some rules.

Differential lines must be kept close to each other, the use of a twisted pair cable is

recommended and the board layout should be done with traces of equal length for the

differential signals. Impedance matching also has to be considered in order to avoid signal

reflections.

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Signal levels

RS-485 drivers must produce a differential output voltage of at least 1.5 V into a 54

Ω load

(usually the lines are terminated at both ends with a 120

Ω resistor; each driver seeing a

120/2=60

Ω load, 54 Ω being the worst case (10% tolerance). Receivers must detect a

differential voltage as low as 200 mV. This is because if a degradation of the signal occurs

across the line, the communication must not be affected. However, the driver differential

output voltage depends on its output current capability. If many receivers are connected to

the line, the driven load seen by the driver increases, as receivers are added to the bus,

requiring more current. The RS-485 standard defines a unit load (UL), which is about a 12

k

Ω resistance over the bus voltage range interval (-7 to +12 V). RS-485 drivers must

generate the required output minimum voltage (1.5 V) with at least 32 unit loads on a

terminated bus with a 120

Ω resistor on each end of the line.

The ST485EB is a half-duplex RS-485 transceiver with a low unit load. A network provided

with these 1/8 UL transceivers allows interconnection of 256 nodes, driven by means of a

standard compliant driver, without overloading it.