Overview; Rs-485 Standard - ABB FMBA-01 User Manual

Overview

Overview
This chapter contains a short description of the RS-485 standard
and the FMBA-01 Modbus Adapter module, and a delivery
checklist.

RS-485 standard

RS-485 is a serial interface standard for communication over a
twisted-pair cable. Because the RS-485 signal transmission is
differential, it provides better protection against noise and longer
transmission distances than the RS-232. RS-485 is a half-duplex
multi-drop network, which means that multiple devices may reside
on line. Only one transmitter may be active at any given time.
The RS-485 standard specifies only the electrical characteristics
of the bus system. The communication protocol and
communication speed depend on the used application. For
example the electrical characteristics of the PROFIBUS and
MODBUS are based on the RS-485 standard.
The RS-485 transmission line consists of two wires, A and B. The
signal transmission is based on the voltage difference between
the wires. Minimum detected voltage difference is 200 mV. The
potential difference between the two wires determines the logic
state bit: when B is at higher voltage than A, the state is defined as
bit 1 (data high) and when A is at higher voltage than B, the state
is defined as bit 0 (data low).
The common mode voltage between RS-485 network devices is
limited to -5...+12 V. Ground wire and cable shield should be
connected to prevent common mode voltage between the network
devices from drifting outside the allowable limits.
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Overview