Best way to document system integrations

Hi All
I'm the product manager for 3 ERP/CRM systems and we have lots of integrations. I've never found a good way of documenting this is a single place. An example would he

System A/Field B Maps to System X/Field Y, however there is a lot of logic so the I need a way to indicate this mapping, the business rules. and logic and any special notes in a single way.

I tried using Visio a while back when I was working on a lot of integrations with Scribe Insight

Anyone got any suggestions of methods/tools?

Thanks

Mark