tkmcgov (Customer) asked a question.
PLC to VFD Modbus Write Help
I am trying to control (run command and frequency reference) an iS7 drive from LS Electric with an H2-DM1E PLC over Modbus/TCP. I am able to use the MRX command in Do-More Designer to successfully read the holding registers, so I know the VFD and PLC are communicating. However, my write command has no impact. I can use a separate program (Baseblock ComTest Pro) to write directly over Modbus (without the PLC) and that successfully runs the drive.
it must be interlock so does first mrx and then when done next instruction mwx. (these instruction take longer than one scan). In dl06 PLC they have a busy bit sp116(your plc may have different bit for this) i use this to do interlock otherwise second instruction will never happen
+1 @AkaHammer (Customer)
Here is a post that will describe the sequencing from one Modbus instruction to the next.
https://accautomation.ca/brx-plc-serial-communication-modbus-rtu-to-solo-process-temperature-controller/
Regards,
Garry
https://accautomation.ca/series/brx-do-more-plc/
Your instruction is currently edge triggered did you try toggling C2?
Contrary to what is being posted here, Modbus TCP instructions do not require interlocking. Modbus RTU does.
I believe your problem is due to the edge triggered MWX/MRX instructions, signified by the little triangle at the end of the rung.
If you toggle C2, it will write the command once. If you toggle C1, it will refresh the V10.. V17 values once.
You may wish to set you MRX instruction to run continuously at an interval, such as 500 ms, and then only MWX once, when V15 does not equal your command of 315.
This ladder is just to test out the commands, not the final use. I have been manually toggling C1/C2 as the MWX/MRX are currently set to edge triggered.
However, that being said, I have resolved the issue. Thank you all for offering potential solutions.
What was the issue?
Modbus TCP-that makes sense that it can handle multiple requests at once but Modbus RTU can not and testing proved that, Serial Com can only handle one request at a time. Thanks for the correction.
And yes what was the issue?