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IN700-485 Series
9. Modbus Interface
9.1. General Description
The Modbus protocol is an application-layer messaging protocol developed by Modicon in 1979. It is used to
establish master-slave/client-server communication between intelligent devices over different types of buses or
networks. The Intesis gateway supports Modbus TCP and RTU.
Modbus is a request/reply protocol and offers services specified by function codes. Modbus function codes are
elements of Modbus request/reply PDUs (Protocol Data Unit).
9.2. Modbus Server Interface
9.3. Description
The gateway acts as a server device in its Modbus interface, which can be:
• The Ethernet port for Modbus TCP.
• The EIA-485/EIA.232 ports for Modbus RTU.
NOTICE
Modbus RTU and TCP modes can be active in the gateway one at a time or both simultaneously.
To access the gateway's points and resources from a Modbus client device, you must specify as Modbus register
addresses those configured inside the gateway corresponding to the signals on the field device protocol.
9.4. Functions Supported
NOTICE
This part is common for both Modbus TCP and Modbus RTU.
Table 4. Modbus functions
#
Function
01
Read Coils
02
Read Discrete Inputs
03
Read Holding Registers
04
Read Input Registers
05
Write Single Coil
06
Write Single Register
15
Write Multiple Coils
16
Write Multiple Registers
If poll records are used to read or write more than one register, the range of addresses requested must contain
valid addresses; if not, the Intesis gateway will return the corresponding Modbus error code.
All registers are of 2 bytes, even if they are associated with signals of bit type on the other protocol's side. Its
content is expressed in MSB .. LSB.
Modbus error codes are fully supported. They are sent whenever a non-valid Modbus action or address is
required.
1
MSB: most significant bit; LSB: less significant bit
Page 14 of 36
Read/Write
R
R
R
R
W
W
W
W
1
Modbus Interface
USER MANUAL Version 1.0
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