120 SI-Ethernet User Guide
Issue: 1
7.2.2 Data structure
Communication between devices is based upon Modbus Application Data Units
(ADUs), the ADU consists of 2 parts, the Modbus Application Protocol (MBAP) header
and the Modbus Protocol Data Unit (PDU).
Figure 7-1 Modbus Data Structure
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
Modbus ADU
Function CodeMBAP Header Data
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
1Slot 1
2Slot 2
3Slot 3
4 Slot 4 (onboard Ethernet)
254 Self
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