Control.com - Nerds in Control

We are working on a project where we are to replace a RS-232 modbus interface with a TCP interface. For this Modicon has recommended the NOE cards, i.e. Ethernet Cards for Quantum PLCs. The customer expects an increase in the communication speed. My point is that even though we have provided a high bandwidth to Modbus but Modbus has its generic limitations. So it will run at the same speed, i.e. 19200 bps over the 100 Mbps channel. Is there a document reference that I can see for the same?

NOE Cards are communicating with the CPU directly by the backplane. There is no serial limitaion or conversion involved. It is Modbus TCPIP protocol and i can assure you it is really fast. That is why you dont want to use an external Modbus TCPIP to serial converter.

The protocol specification documents are available from: http://www.modbus.org/specs.php
MODBUS APPLICATION PROTOCOL SPECIFICATION
MODBUS MESSAGING ON TCP/IP IMPLEMENTATION GUIDE

You will see that a Modbus message is transported via IP over Ethernet, a.k.a tunneling. The Modbus message is carried in the payload of the IP. The throughput is limited by the same factors that limit any communications using IP over Ethernet.

There is a processing delay introduced by the receiver of the message before a response is returned.

In a nutshell, with the exception of the format of the data carried in the IP payload being Modbus,
the network, transport, and physical layers have nothing in common with Modbus over serial line.

That being said, if somewhere in the end-to-end path between communicating nodes, a serial line
exists, throughput will, of course, be limited.

I have used NOE to HMI (PC Ethernet port) and NOE to NOE and performance is very good.

Bill

For most Modicon device you will see a considerable increase in communications because this is native ethernet, althought the packet size is still small. Some 3rd party manufacturers have taken the approach of mothering a Modbus Ethernet to serial converter to their serial communications cards. Even with these there is typically an increase in performance because the bus speed is usually much faster than the serial speed. The only complaint I could have is that they kept the communicaitons packet size the same as the serial packet size.

Fred Loveless
Application Engineer
Kepware Technologies
http://www.kepware.com