Modbus Serial vs TCP and DNP3 Serial vs TCP

Difference between Modbus Serial and Modbus TCP and DNP 3.0 Serial vs DNP 3.0 TCP... very straight forward explanation..

taken from...

http://controltoolbox.com/blog/2009/03/why_dnp_over_ip_is_smarter_tha.html

There are many aspects of DNP3 protocol that are superior compared with Modbus. However, I like to talk about DNP/IP and Modbus TCP specifically. Like Modbus, DNP was originally designed to cover a serial link. As IP becomes more popular, both DNP and Modbus add IP option to the core protocol. What DNP designers did was to just simply embed the DNP traffic in an IP envelop. Modbus designer modified the header and removed the CRC of the traditional Modbus message to migrate to Modbus TCP.  I think they did that because they were thinking that TCP header already has a CRC and keeping the Modbus CRC is kind of redundancy.

Now, here is the problem with Modbus TCP. Lots of people have Modbus TCP masters and legacy Modbus devices. For them, connecting the master to the device requires more than a simple terminal server that converts IP traffic to serial. The reason is there would be a need for a gateway that can adjust the protocol from serial version to TCP version and vice versa. That’s why you see many manufacturers making devices called Modbus gateway. The situation with DNP is much simpler. Since the DNP packets in IP and serial version are the same, a simple terminal server can do the job. Terminal server gets the IP traffic and sends the DNP payload on the serial side. Conversely, it gets the DNP packets on the serial link and embeds in the IP envelopes to transmit over the IP link.