Typical Bacnet Eia-485 Data Bus Transmission Example - Distech Controls ECL/ECB-PTU Series User Manual

APPENDIX A
Typical BACnet EIA-485 Data Bus
Transmission Example
The graph below shows electrical signals for the transmission of one byte of data (hexadeci-
mal D3, least significant bit first). Note that when idle, the BACnet MS/TP data bus is biased
such that the Net + connection is positive with respect to the Net - connection.
Figure 72: Example of a Typical EIA-485 Data Bus Transmission
Net +
Mark
Net -
Idle
The permitted voltage levels of an EIA-485 transmitter and receiver are shown below.
Figure 73: Typical EIA-485 Data Bus Operating Signal Levels
EIA-485 Transmitter Signal Voltage
+12 Volts
+2 Volts
Voltage V
Net-,Net+
-2 Volts
-7 Volts
Network Guide

Typical BACnet EIA-485 Data Bus Transmission Example

Mark Space
1 1 0 0 1 0
+12 Volts
Permissible
Range
+0.2 Volts
Voltage V
-0.2 Volts
Permissible
Range
Mark
Mark
Idle
1 1
EIA-485 Receiver Differential Input
Net-,Net+
-7 Volts
Maximum
Operating
Transition
Range
Region
129