The underlying modbus node package does not support custom FCs.
Your choices are...
- raise a "Feature Request" on the github repo + cross your finger
- fork, add "custom FC feature", publish a PR (pull request), wait for developer to merge an release.
- fork, add "custom FC feature", publish an alternative node in NPM.
- DIY it - here is the basis for a DIY solution
As for your error...
You have asked Buffer Parser to use 6 bytes (but only supply it with 4 bytes)
The proper solution to convert a buffer of bytes into an array of integers is to use the length -1 (it is documented in the built in help)
Y
