CARLO GAVAZZI UWP 3.0, Руководство пользователя, Страница: 42 / 45

How to install an UWP 3.0 system
42
3
Appendix “A” – RS485 network guidelines
3.1 MODBUS via Serial line (RS-485) guidelines
3.1.1 Introduction
The RS-485 is a half-duplex multidrop network: multiple transmitters and receivers may share the same
line, but only one transmitter may be active at any given time. The TIA/EIA-485-A says nothing about the
communication protocol to be used.
The MODBUS standard defines an application layer messaging protocol. The MODBUS Serial Line is a
master-slave protocol which may use the RS-485 as its physical interface.
3.1.2 RS-485 cable
The suggested cable is the shielded twisted for three wire connections. Cables must be at least 0,5mm
2
.
The typical cable impedance should be between 100 ohm and 120 ohm.
3.1.3 RS-485 grounding
Communication through a RS-485 system is made by means of a balanced pair with a common so a
three wire connection is needed. The shield must be connected directly to earth ground, preferably at one
point only for the entire RS-485 Bus. Generally this point should be chosen on the master device or on
its tap and not shared with other devices (inverters,…) which could add external noise or disturbance.
3.1.4 RS-485 shielding
A shielded cable is required to guarantee a high degree of immunity to EMC and lightning events.
The improvement of EMC immunity can also be done connecting the minus of the DC power supplier of
the system (-) to the earth ground.
3.1.5 RS-485 termination
A termination is needed at each end of the bus. Termination must not be placed anywhere else.
For
the relevant termination please check the official documentation here:
www.modbus.org/docs/Modbus_over_serial_line_V1.pdf
3.1.6 RS-485 wiring procedure
Suitable wire sleeves must be used for wiring. If any wire splice is to be used, this splice must be soldered,
taking special care to cover the splice with a cable shield and to ensure proper continuity.