Table of Contents
Advertisement
30
About the 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.
RS-485 bus cable should be terminated with a 120 ohm resistor on both ends to
prevent signal reflection. When no device on the network is transmitting, noise can
be falsely interpreted as communication data. To avoid this, termination can be
included with a fail-safe circuit (pull-up and pull-down resistors). The circuit forces
the bus into a known idle state when no device is transmitting.
Recommended cabling for RS-485 buses
It is recommended to use shielded twisted pair cable, impedance 100...150 ohm
with 120 ohm termination resistors. Belden 9842 cable or equivalent is
recommended. Belden 9842 is a dual twisted pair cable with a wave impedance of
120 ohm.
Bus termination is required to prevent signal reflections from the bus cable ends.
Termination should be activated on devices located at bus ends and deactivated on
other devices.
Possible drive interfaces
Generally there are four possible ways that the drive can be connected to SREA-01:
• Using the panel port,
• Using an embedded Modbus connector,
• Using an add-on Modbus adapter such as FMBA-01, FSCA-01 or RMBA-01,
• Using an add-on Ethernet adapter such as RETA-01, RETA-02 or FENA-01 and
Modbus/TCP.
Installation of the drive connection
Advertisement
Table of Contents
