GE Industrial Solutions WattStation Pedestal EVSE Charger MODBUS Register Mapping Guide - Manual (Page 6)

WattStation TM EV Charger MODBUS Register Guide
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2.2 MODBUS/TCP Overview
MODBUS communication over TCP has already become a de-facto automation standard. Figure 2.4
shows the MODBUS/TCP ADU format.
MODBUS/TCP ADU format.
Figure – 2.4
A dedicated header called the MBAP header (MODBUS Application Protocol header) is used to identify
the MODBUS/TCP Application Data Unit as Figure 2.5.
MODBUS/TCP MBAP Header
Figure – 2.5
The MBAP header is 7 bytes long:
• Transaction Identifier - It is used for transaction pairing. The MODBUS Client, MODBUS
Gateway, and the MODBUS server will use it to correspond the response to the
corresponding request. The MODBUS server MUST copy in the response the transaction
identifier of the request.
• Protocol Identifier – It is used for intra-system multiplexing. The MODBUS protocol MUST
be identified by the value 0.
• Length - The length field is a byte count of the following fields, It includes the Unit
Identifier and data fields.
• Unit Identifier – This field is used for intra-system routing purpose. It is typically used to
communicate to a MODBUS or a MODBUS+ serial line slave through a gateway between
an Ethernet TCP-IP network and a MODBUS serial line. This field is set by the MODBUS
Client in the request and
M MU USSTT be returned with the same value in the response by the
server. In this architecture design, the unit ID is ignored by the EVSE.
Function Code Data MBAP Header
MODBUS/TCP ADU
Transaction ID Protocol ID Length Unit ID