I am new to Modbus (normally program in kmdigital) and would really like to get my feet wet in this area. I am having a hard time locating good resources to study, looking for something like "Modbus for Dummies" or such. Any help is appreciated.
You should find this page helpful:
Try the following where some else just asked the same question:
This answer was supposed to be attached to that thread as well, but seems to have gotten separated into a thread if its own.
Also, the control.com web site has a very good search function. If you search on that for "modbus", you will get lots of results. Modbus is a popular protocol, and questions about it are common, I just tried this out of curiosity, and got over 13000 matches.
After you have read some of the forum responses, have a look at the official specifications. Unlike most specs, these ones are rather well written and understandable. Also unlike most industrial protocols Modbus is an open protocol so there is lots of information available on how it really works.
I would pay particular attention to what the official spec has to say about data table addresses. There seems to be quite a lot of simply *wrong* information about this in many places. The spec itself however explains it quite well.
Modbus 101 - Introduction to Modbus
Chipkin's Modbus for Field Technicians
draft of Ron Hulsebus' forthcoming book - Modbus: The Manual
Introduction to Modbus using (National Instruments) Labview
Modbus Troubleshooting guide
Simply Modbus About Files