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9. Appendix
9. 1. 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 all these characteristics. They are also
summarized in the next sections (Modbus 2-wire and 4-wire standard schematics). The latest generation Telemecanique devices
(Altivar 31, Altivar 71, etc.) conform to this specification.
Older devices comply with earlier specifications. The two most widespread are described in the following sections:
• "Uni-Telway schematic" page 42
• "Jbus schematic" page 43
Requirements enabling different types of protocol to coexist are given in the following section in this appendix:
• "Creating a Modbus bus using non-standard equipment" on page 45