A.2 Termination
You can reduce reflections that cause communication and data errors on RS-485/RS-422 networks by
terminating a data cable with a value equal to its characteristic impedance. Although termination is often
unnecessary on networks where the baud rates are slow or the cables are short, termination becomes
important as the baud rate increases.
Resistors acting as terminators typically have 120-130 ohms, although twisted-pair cable impedances can
be as low as 100 ohms. Liebert recommends 120 ohm terminating resistors on RS-485/RS-422 networks.
You must apply a value that closely matches the cable impedance as near as possible to the ends of the
network segment.
NOTE: Some third-party manufacturers provide a resistor within a device, using a jumper to disable the
termination option if termination is not required. If you are using one of these devices, make sure that
only devices that require termination are set to have termination.
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.
Vertiv | Liebert® SiteLink-E™ Installation Manual | 24
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