There are several classes of service that are part of the Modbus Application Protocol. Each of these classes is described below.
Read/write both discrete and analog data values from PLC register files.
Services make relatively minor alterations to ladder logic programs with a highly controlled introduction of these changes into the executing program.
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 services allow the user to define parameter values which affect the PIC’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.
