Trying to get a modbus lib to work on the iPhone to make a generic free application that would work with all devices that would support MODBUS TCP/IP.

Is there any library that already exists?

 

Try this one:
https://launchpad.net/libmodbus

 

(you will have to remove the spaces from the URLs below)

We also have a portable Modbus library.
http://pvbrowser.org/pvbrowser/sf/manual/rllib/html/classrlModbus.html

See: http://pvbrowser.org

But i would prefer a web application that runs within the normal web browser. Thus you could access from all devices that provide a web browser not only the iPhone.

Create a webpage on your server that has access to modbus and display the values in the browser. We use the following architecture:

http://pvbrowser.de/pvbrowser/pic/prinzip.png

The modbus values are read from a shared memory and outputs are send over the mailbox. Your web application can now use it. You can use our Modbus daemon which 1st configured by an INI-file.

All you need is a language binding for our class

http://pvbrowser.org/pvbrowser/sf/manual/rllib/html/classrlModbusClient.html

You can create a PHP binding for example with
http://swig.org

How this is done for python you can see in directory
pvb/language_bindings from our
http://pvbrowser.de/pvbrowser/tar/pvb.tar.gz
PHP is almost the same.

An alternative to a language binding is to implement the Modbus access in C++ as a separate application that is called from PHP function passthru()