Frames Structure - Scaime eNod4-F ETHERNET Software User Manual

eNod4 supports Modbus TCP (or Modbus TCP/IP) protocol: a document Modbus Messaging on TCP/IP
implementation guide provided by Schneider Automation outlines a modified protocol specifically for use over
TCP/IP. The official Modbus specification can be found at Modbus organization (www.modbus.org).
ADU (Application Data Unit) and PDU (Protocol Data Unit): aside from the main differences between serial and
network connections stated above, there are few differences in the message content between Modbus TCP and
Modbus RTU.
Starting with Modbus RTU frame (ADU), the checksum disappears. From now on data integrity is granted by Ethernet
Data Link layer. Slave ID address is suppressed and supplanted by an identifier (Unit ID) that is a part of a
complementary data header called MBAP (Modbus Application Protocol) header. The MBAP header is 7 bytes long.
MBAP header: fields are defined below:
Length
fields
(bytes)
Transaction
2
Identifier
Protocol
2
Identifier
Length 2
Unit Identifier 1
Supported functions: identical to Modbus RTU ones.
•
Read multiple registers* :
•
Write single register* 06
•
Write multiple registers*
*1 register = 2 bytes
Maximal number of registers = 123

5.3 Frames structure

•
By default and as in Modbus RTU, during a read or write transaction, the two bytes of a register are
swapped. The MSB is transmitted first and then the LSB. However it may be possible using eNodView
software to invert the swapping of data in a register.
•
if a data is coded on 4 bytes (that means it requires two registers) , the two LSB are stored in the low
address register and the two MSB are stored in the high address register Modbus RTU request command
example sent to the slave in hexadecimal:
Description
Transaction pairing
(request / response Modbus)
0 = MODBUS Protocol
byte count of the remaining fields
(Unit ID + Function Code + Data)
Idendification of a remote server
(non TCP/IP or other buses), 0x00
or 0xFF otherwise
03 / 04
H H
H
10
H
18/113
User manual SCAIME:
NU-Soft-eNod4F-ETH-E-0716_235709-C.doc
Client (Master) Server (Slave)
Echoed back by the
Initiated by the Client
Server
Echoed back by the
Initiated by the Client
Server
Initiated by the Client Initiated by the Server
(request) (response)
Echoed back by the
Initiated by the Client
Server