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{"searchBar":{"inputPlaceholder":"Search by keyword or ask a question","searchBtn":"Search","error":"Please enter a keyword to search"}}M340 specific system words, %SW138 to %SW141
Goals and Symptoms
To facilitate the migration from LL984 to M340, four new System Words have been added to offset the starting address of variables which are accessed by Modbus requests.
If this functionality is being tested in the simulator, it is important to note that it was not added to the simulator until Unity Pro version 13.1.
Facts and Changes
| System Word | Modbus request impacted | First accessed variable when Starting Address = 0 |
| %SW138 Modbus Discrete Input Offset | FC02 Read Discrete Inputs | %M[%SW138] |
| %SW139 Modbus Coil Offset | FC01 Read Coils FC05 Write Single Coil FC15 Write Multiple Coils | %M[%SW139] |
| %SW140 Modbus Input Register Offset | FC04 Read Input Registers | %MW[%SW140] |
| %SW141 Modbus Holding Register Offset | FC03 Read Holding Registers FC06 Write Single Register FC16 Write Multiple Registers FC22 Mask Write Register FC23 Read/Write Multiple Registers | %MW[%SW141] |
The base address is the number of %M or %MW which correspond to Modbus address 0000.
Examples:
-
- %SW140 contains 1000. When the Modbus server receives a Read Input Registers request (code 04) with starting address equal to 0000, it returns values from %MW1000.
%SW138 contains 0001. When the Modbus server receives a Read Discrete Inputs request (code 02) with starting address equal to 0000, it returns values from %M1.
- %SW140 contains 1000. When the Modbus server receives a Read Input Registers request (code 04) with starting address equal to 0000, it returns values from %MW1000.
Warning : it is mandatory to set the value of those 4 words with Initial value mechanism, rather than writing in an application section. Otherwise, upon PLC Init command or setting %S0 to 1 or downloading application, the 4 system words are set to 0000 and the behavior of Modbus server is wrong.
Causes and Fixes
This information is included in the Unity Pro help on line documentation as of version 7.0
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