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Appendix
62 S1A53844 01/2011
RS485 standard
The RS485 standard (ANSI/TIA/EIA-485-A-1998) allows variants of certain characteristics:
• Polarization
• Line termination
• Distribution of a reference potential
• Number of slaves
• Length of bus
It does not specify the connector type or pinout.
The Modbus specification published on www.modbus.org in 2002 contains precise details of these
characteristics. They are also summarized in the next sections (Modbus 2-wire and 4-wire standard
schematics). The latest generation Schneider Electric devices conform to this specification.
Modbus 2-wire standard schematic
The standard schematic corresponds to the Modbus specification published in 2002 on www.modbus.org
(Modbus_over_serial_line_V1.pdf, Nov 2002) and, in particular, to the 2-wire multipoint serial bus schematic.
Schematic diagram
1n F
650 Ω
650 Ω
120 Ω
1n F
120 Ω
5 V
0 V
G
R
G
R
G
R
D1
Common
D0
Slave 1
Slave n
Master