20 Network Guide
Data Bus Topology and EOL Terminations
Function of EOL Terminations
The first and last device on the data bus must have End-of-Line (EOL) termination resistors
connected across the two data lines/wires of the twisted pair. These resistors serve the follow-
ing purposes:
• EOL terminations dampen reflections on the data bus that result from fast-switching (high-
speed rising and falling data edges) that otherwise would cause multiple data edges to be
seen on the data bus with the ensuing data corruption that may result. The higher the
baud rate a data bus is operating at, the more important that EOL terminations be properly
implemented. Electrically, EOL terminations dampen reflections by matching the imped-
ance to that of a typical twisted pair cable.
• EIA-485 data bus transmitters are tri-state devices. That is they can electrically transmit 1,
0, and an idle state. When the transmitter is in the idle state, it is effectively offline or dis-
connected from the data bus. EOL terminations serve to bias (pull-down and pull-up) each
data line/wire when the lines are not being driven by any device. When an un-driven data
bus is properly biased by the EOL terminations to known voltages, this provides increased
noise immunity on the data bus by reducing the likelihood that induced electrical noise on
the data bus is interpreted as actual data.
When to Use EOL Terminations
EOL terminations should only be enabled / installed on the two devices located at either end of
the data bus. All other devices must not have the EOL terminations enabled/installed.
Figure 2: EOL Terminations Must be Enabled at Both the First and Last Device on the Data Bus
Devices with built-in EOL terminations are factory-set with the EOL termination disabled by
default.
The BACnet/IP to MS/TP Adapter does not have EOL Termination (and BACnet MS/
TP Data Bus biasing) capabilities to be used at the end of a BACnet MS/TP data
bus. Instead, use the BACnet/IP to MS/TP Router for this application.
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