ControlNet and Modbus

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Thread Starter

Darker

Hi,

I am currently seeing control system architecture for one of the projects, and there is something that I don't understand.

Summarize it a bit:

Level Transmitter is connected to a data hub via 2 connections a TRL/2 Field bus, and a Modbus.

The TRL/2 Field Bus goes to a field bus modem and to the operator.

The Modbus goes to the I/O Rack, and the I/O Rack links with the Processor (PLC) via ControlNet.

The I/O Rack links to the Pakscan via Modbus.

I understand the advantages of ControlNet over other communications links such as DeviceNet and Ethernet as it provides scheduled data on time (real time).

But What I don't understand why can't we link I/O Rack and the Data hub via a ControlNet instead of Modbus.

The PLC will get the information in real time, but the I/O Rack won't get the information/data in real time, then the PLC is not getting info/data in real time.

Thank you in advance.
 
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Lynn August Linse

I am sure you can - just buy a unit which was the more expensive baseband technology of ControlNet, plus the protocol stack is dozens of times more complex for a devcie maker to use (I've worked with CIP on Ethernet and serial, but not ControlNet - should be similar). A vendor making a ControlNet device is boxed into Rockwell Automation markets - most of which now want Ethernet, not baseband.

No one is forcing you to buy Modbus, but then ... why not use PROFIBus or PROFINET? Why not BACnet or DNP3? Why not REST and XML?

In the end it is market driven. I will suggest that the added cost of ControlNet means volumes are lower & customer base smaller, whereas Modbus is cheaper (to engineer for device maker) and has a wider market appeal.
 
> I am currently seeing control system architecture for one of the projects, and there is something that I don't understand.

...Snip...

Simple. To talk to those devices that don't speak ControlNet. If you can avoid Modbus, then by all means do.

It may be the hacked-up illegitimate child of Morse code and Pig Latin, but anything worth communicating with, communicates in Modbus. :)
 
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