"I want to design a data aquisition card with 8 channel input (for example 8 of 4-20mA)and a RS485 bus with MODBUS protocol as output. "

And with 2 or 3 way isolation.

Well this is really a job for a organized design house to say the least. Maybe you can do the design but to certify its specifications is a nightmare for an individual. And if you think, by designing it yourself you will save cost, you need to think again -some of these products are available at a fraction of the cost from Taiwanese sources compared to what you get from giants like ABB, Siemens, Yogokawa etc. Just spend some time googling.

If you are purely doing it for hobby, then go ahead and post the query in other sites like www.control.com or www.modbus.org which are more relevant in this case compared to 8052.com

Raghu