Appreciate for your interest in ABB ac500 PLC, at this stage recommended to go through this video's how to ? on you tube @
You can find more details under downloads around 99 manuals.
More over in associated app here you can find example code for ac500 + mpodbus interface.
The videos alrady linked provide a good description of and introduction to the AC500 PLC's. You can also find many code examples at http://www.abb.com/product/ap/db0094d...
But, it's time for you to learn another programming language. While you can "make a serial link work" using ladder logic all you are really doing is using ladder to place function blocks on a page. The resulting code can be very hard to follow and difficult to debug. You will find with practice that FBD is a MUCH easier language to use than LD - and also very usefull with other vendors' PLC's