Typical BACnet EIA-485 Data Bus Transmission Example
Network Guide
129
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
The permitted voltage levels of an EIA-485 transmitter and receiver are shown below.
Figure 73: Typical EIA-485 Data Bus Operating Signal Levels
Net
-
Net
+
Mark
Idle
Mark
Idle
Sp
ac
e
S
ta
rt
St
op
1
0
1
0
0
1
1
1
Mark Space
Ma
rk
Sp
ac
e
Mark
+12 Volts
+2 Volts
-2 Volts
-7 Volts
Permissible
Range
Permissible
Range
Voltage V
Net-,Net+
EIA-485 Transmitter Signal Voltage
+12 Volts
+0.2 Volts
-0.2 Volts
-7 Volts
Maximum
Operating
Range
Transition
Region
Voltage V
Net-,Net+
EIA-485 Receiver Differential Input
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