Hi David,
I enjoyed reading your post because I am currently working on similar project with AccuDc 243 series . I am trying to read V, I, P. Using a max 485 with arduino uno, schematics are the same as yours. The connection is correct, however the code that I have , not quite confident. Currently when I run the code, I am all getting the default response from max 485 which means I am not receiving anything from the accudc power meter. I tried to run your code and I am running into similar problem the people who posted before me. The code doesn't compile.
Is it possible to share the "simpleModbusmaster.h" and "simpletimer.h" library from your side ?
I look forward to hearing from you.