Eos –Gate Installation and operation manual
All the information provided with this document are property of Carlo Gavazzi Controls SpA
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10 Appendix
10.1 MODBUS over Serial line (RS-485) guidelines
10.1.1 Introduction
RS-485 is a half-duplex multidrop network: multiple transmitters and receivers may reside on 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 RS-485 as its physical interface.
Please refer to the “Modbus over serial line V1.02” specification for further information.
The following picture represents a typical MODBUS over serial line system.
10.1.2 RS-485 cable
The suggested cable is the shielded twisted pair for 2 wires connections (2W-MODBUS) and the
shielded double twisted pair for 4 wires connections (4W-MODBUS). If there are not any reason to
use the 4W-MODBUS (e.g. an existing serial line ) a 2W-MODBUS based system is the right
choice because the Eos-Array products are based on a 2 wires system. The following notes are
related to 2W-MODBUS systems.
Cables must be at least AWG 24. The typical cable impedance should be between 100 ohm and
120 ohm.
A specific cable for MODBUS RS-485 application must be chosen.
Code Description
IDv
Derivation
interface
(Passive Tap)
ITr
Interfance with
the trunk
AUI
Attachment unit
interface (Active
Tap)
LT
Line termination