Emerson SiteLink-12E - Manual (Page 20)

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A.3 Bias
Data collisions occur when two devices enable their transmitters at the same time; these are likely to
occur on RS-485/RS-422 networks. A master/slave protocol has one master and many slaves. The
slaves are always listening and respond only when they hear their address in the master’s request for
information. When the master is not transmitting, the network will float, enabling noise to falsely
trigger one of the slaves’ receivers, since the receiver’s output is undefined when the receiver’s input
voltage is between 200mV and -200mV (known as the undefined state).
You can apply bias to the network to ensure that the network assumes a defined state even when all
device transmitters are off.
When bias is used with terminators, the float network state’s voltage exceeds 200mV. This means
that:
• The receivers are biased in the mark state (OFF, logic 1) when the network is idle or when the
transmitter sends a logic 1.
• During the transmission of a logic 0 (less than -200mV), the network will be in the space state
(ON, logic 0).
Liebert’s DIAG boards can serve as bias on RS-485/RS-422 networks. Apply bias in the middle of the
network, and apply termination only at the two end devices.