Vertiv Liebert SiteLink-12E, Installation Manual

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Figure 4.2 Liebert precision cooling units networked with Liebert SiteLink-E modules
4.2 RS-485 and ARC156 Wiring Considerations
An RS-485 network is intended to be configured as a linear bus with daisy-chained connections (star
topologies are not recommended). Termination in RS-485 and ARC156 setups is usually applied to both
ends of the network (see
Wiring Port S1 for RS-485 Network—Four-Wire on page 16).
Repeaters often are required when connecting 32 or more RS-485 or ARC156 devices or when using
ARC156 cable segments longer than 2000 feet. Refer to the BMS documentation for information on when
to use repeaters.
To reduce communication and data errors, terminate each end of the network with a resistor whose value
equals the network's characteristic impedance. Some third-party manufacturers provide a built-in resistor
that you enable or disable with a jumper. Make sure that only devices at the end of a network have
termination enabled.
EXAMPLE:
If the network’s characteristic impedance is 120 Ohms, terminate the network by
placing a 120 Ohm resistor across the Net+ and Net- connectors of the Liebert SiteLink-E and
a 120 Ohm resistor across the + and - connectors of the furthest third-party device.
Bias must be applied to each RS-485 network segment. To add bias to a network segment, put
the bias jumper in place on the DIAG485 that is in the middle of the segment. You can use
additional DIAG485s with the bias jumper removed to monitor network communication, but
only one DIAG485 may have the bias jumper in place.
Vertiv | Liebert® SiteLink-E™ Installation Manual | 14