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FnIO MODBUS Adapter NA-9189
FnIO S-Series
Copyright(C) CREVIS Co.,Ltd Support +82-31-273-6453 URL : www.crevis.co.kr
6. MODBUS/TCP INTERFACE
6.1.
MODBUS/TCP Protocol
The MODBUS messaging service provides a Client/Server communication between devices connected on an Ethernet
TCP/IP network. All MODBUS/TCP messages are sent via TCP on registered port 502.
6.1.1.
Comparison of MODBUS/TCP And MODBUS/RTU
This header provides some differences compared to the MODBUS RTU application data unit used on serial line:
The MODBUS ‘slave addresses’ field usually used on MODBUS Serial Line is replaced by a single byte ‘Unit
Identifier’ within the MBAP Header. The
‘
Unit Identifier’ is used to communicate via devices such as bridges,
routers and gateways that use a single IP address to support multiple independent MODBUS end units.
All MODBUS requests and responses are designed in such a way that the recipient can verify that a message is
finished. For function codes where the MODBUS PDU has a fixed length, the function code alone is sufficient. For
function codes carrying a variable amount of data in the request or response, the data field includes a byte count.
When MODBUS is carried over TCP, additional length information is carried in the MBAP header to allow the
recipient to recognize message boundaries even if the message has been split into multiple packets for transmission.
The existence of explicit and implicit length rules and use of a CRC-32 error check code (on Ethernet) results in an
infinitesimal chance of undetected corruption to a request or response message.
MODBUS/TCP
MBAP Header
Function
Data
7 chars
1 char
Up to 252 chars
MODBUS/RTU
Start
Address
Function
Data
CRC Check
End
≥
3.5 chars
1 char
1 char
Up to 252 chars
2 chars
≥
3.5 chars
Function and data field of MODBUS/TCP are identical to function and data field of MODBUS/RTU.
