I want to develop RS-485 Modbus communication between Yokogawa DCS and Emerson Flow computer to read and write data. At same time I want RS-485 Modbus communication of Flow Computer with Gas chromotograph to read data. Flow computer has only one RS-485 port. How can I develop it?

Naeem Tahir

 

Hello Naeem;

A Modbus network running on RS-485 is multidrop, so a master can access two slaves or more. Each slave must be parametrized with a unique address on the bus (0-255), and the master will address his requests to each slave using that address to distiguish between them.

Now, in a master-slave network, only the master can initiate a call. So if data mut be recieved in the flow computer coming from the chromatograph unit, then the Yokogawa DCS master must first read the chromatograph's registers (Modbus function 3 or 4), then write (Modbus function 6 or 16) these values to the flow computer (as long as the flow computer's design has spare registers for this purpose, or an area developped by the manufacturer for this type of data exchange). A slave's memory area for Modbus exchanges is generally not scalable, so you must check the vendor data on the unit to see if it is possible.

I am not a Yokogawa programmer myself, I hope others can chime in for detailed info on the implementation of Modbus master configuration on that platform.

Hope this helps,
Daniel Chartier

 

You need to configure the flow computer as Master and DCS & GAS Chromotograph as slaves. Give the slave address '1' for DCS and '2' for chromatopgraph and connect them in daisy chain configuration with Flow computer.

 

Mr. Naeem,

You have purchased a Modbus communication package (ALR111, ALR121) for your Tullow Oil Development at chachar sindh. Kindly let me know the Emerson Flow computer model so that I can help you in RS-485 communication.

Regards,

Professional

 

I do not fully understand your problem, but, please look at our products that can either transform a single Modbus slave port to multiple ports, that is you can have multiple Modbus Masters polling a single slave http://www.miille.com/din366-10p.pdf or our Omnii-Comm product http://www.miille.com/din266-p00.pdf that can be configured in many different ways to move data. This product could be the one you need.

For instance, you could configure it to have a Modbus Master port that connects to the Flow Computer and a Modbus Slave port that connects to the Yokagawa DCS that transparently passes thru messages that do not match the Omnii-Comm address to the Flow Computer. A third port could be configured as another Modbus Master to periodically move data between the Gas Chromatograph and the Flow Computer. There are many other configurations possible.

Please contact me directly at j miille @ miille dot com (without all the extra spaces) if you have questions.

Jerry Miille