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136
SI-Ethernet User Guide
Issue: 3
Table 7-2 MBAP Header
The unit identifier within the MBAP header is used to identify whether the destination node is the
host drive or an option module (not available on the onboard Ethernet interface with firmware
versions prior to V01.02.01.10).
Table 7-3 MBAP Unit Identifier
7.2.3
Data access
Data access using Modbus TCP/IP takes the form of a request for data by the master, followed by
a response from the slave indicating success or failure. If no response is received this indicates
that the message has not been received or the message is invalid or the node is unable to reply.
Each drive or option module parameter is internally mapped to a single 16-bit Modbus register, all
Modbus function codes access 16-bit registers only. To access a 32-bit parameter, two contiguous
Modbus registers must be specified in the request and the 32-bit data access scheme must be
used.
7.2.4
32-bit data access
Standard Modbus registers are 16 bits in size and reference a single drive/option module
parameter. To access a 32-bit data value the multiple read/write services must be used to transfer
a contiguous array of 16-bit registers. To instruct the client to select either 16-bit or 32-bit access
bit 14 of the register address is used.
Field
Length
(Bytes)
Description
Transaction Identifier
2
Uniquely identifies the transaction (0 to 65535)
Protocol Identifier
2
Identifies the protocol (0 = Modbus)
Length
2
Number of following bytes in the message
Unit Identifier
1
Uniquely identifies the destination node (0 to 255)
Unit Identifier
Destination
0 or 255
Drive
1
Slot 1
2
Slot 2
3
Slot 3
4
Slot 4 (onboard Ethernet)
254
Self
Bit b15 of the register address is reserved for future use.
NOTE