Appendix

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

Master

R

G

5 V

 

650 Ω

650 Ω

0 V

 

D1

120 Ω

120 Ω

1n F

1n F

 

D0

 

Common

R

G

Slave 1

R

G

Slave n

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Schneider Electric Altivar 212 manual RS485 standard, Modbus 2-wire standard schematic, Schematic diagram