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Old October 30th, 2007, 10:41 AM   #1

sandhya123 is offline
Join Date: Oct 2007
Location: sINGAPORE
Posts: 1
PLC Integration with DCS

Hi PLC Experts,

I am a frequent visitor to this site. Though I do not have any experience in PLCs, I have learnt a lot from this site. Couple of years ago I used to post questions on HVAC controls and I did get pretty useful answers from many of you, for which I am very grateful.

Today I need to solve a problem and hence I am bouncing back to this site. I am sure that I will be getting a help again from this site.

I am working on a batch control project, which is based on S88 standard. I will have to integrate six PLCs with an Emerson DCS.Emerson DCS in this project has six serial link interface cards, with 50 points allotted for each PLCs. The protocol is MODBUS RTU over RS 485. Earlier Emerson DCS had included multidrop communication, which I changed to Point to Point.

Now one of the PLC vendor(Process Equipment Supplier) claims that this will not work as the data transfer would be very slow. My argument is that it should not as the points are just 50. The communication is between PLC(Siemens PLC) and Emerson Controller(MD Plus). I can not change the MODBUS RTU protocol to MODBUS Over TCP/IP as the contract with Emerson DCS was made before I could join the project.

The purpose of communication between PLC and Emerson DCS is to use the transferred data from the PLC in phase logic that will be built in the Emerson Controller. Hence appreciate if you could let me know whether the data transfer rate would be slow using MODBUS RTU over RS 485, Point to Point comm.Is there any other soltuion? Am I missing some thing?

Thanks in advance


Last edited by sandhya123; October 30th, 2007 at 10:46 AM. Reason: Too many double spaces
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Old October 30th, 2007, 10:48 AM   #2
United States

SLC_Integrator is offline
Join Date: May 2007
Location: Salt Lake City, Utah
Posts: 415
Simple asnwer is yes, Modbus RTU is slow.
Modbus TCP is lightning fast compared to RTU.
RTU is usually set to 9600 baud and if your looking for time-critical data transfer, your going to have problems.
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