Appendix C Quantum Ethernet TCP/IP Modbus Application Protocol
Modbus Application Protocol Service Classes
Introduction
There are several classes of service that are part of the Modbus Application
Protocol. Each of these classes is described below.
Data Access
Read/write both discrete and analog data values from PLC register files.
Online
Programming
Services make relatively minor alterations to ladder logic programs with a highly controlled introduction of these changes into the executing program.
Image Download/
Upload
Image download services support the downloading of a ladder logic control program to the PLC. Image upload services support the uploading of a ladder logic control program from a PLC to PC host for archival/backup purposes.
Configuration
Configuration services allow the user to define parameter values which affect the
PLC’s register files, I/O map, communication port configuration and scan attributes, to name a few.
Device
Execution State
Control
The class of service allows the user to start/stop the PLC scan execution. These services require the user to be in an application login context which is obtained through other Modbus services.
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