I finally found something that explains Modbus in a way that I could understand. I hope this will help others too. It is a guide called Modbus for Field Technicians
http://www.modbusbacnet.com/includes/pdf/MODBUS_2010Nov12.pdf
This is an excellent paper !
It definitely explains some things about typical modbus implementations that I don't understand either...
Like the 5 digit addressing. (with the leading 4) Well, I guess I didn't care to know about that since I knew how the
internals of modbus work so I never got into these intricacies.
Thanks for this link !
boB
That link is dead. Anyone got that PDF cached?
Try this link
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjq4enb9_zLAhXosIMKHWNxC04QFggdMAA&url=http%3A%2F%2Fwww.chipkin.com%2Ffiles%2Fliz%2FMODBUS_2010Nov12.pdf&usg=AFQjCNEsKh7XykgNueYnHqONrL2qrIDhNw&sig2=y2ppBRQF2-P76OIzLKZWCg
I tried to attach pdf but it was too large - if link doesn't work I will get it to you another way .
Larry