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Old December 26th, 2002, 08:52 AM   #1
estover
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Siemens S7 w/ Modicon I/O ???

I am preparing to upgrade our plant HMI and PLC systems. Currently we use Modicon 984-785E PLC and Modicon remote I/O. Due to HMI considerations, I really want to use Siemens software which requires Siemens PLC. I have been told its possible to keep the existing Modicon I/O and talk through the 785E to the S7? Due to the costs associated with changing the I/O to Siemens, this solution would be nice.

Can anyone offer any information on this subject?
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Old December 26th, 2002, 10:54 AM   #2
Jim Dungar
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Most HMI packages have drivers that allow communication to almost all major PLC vendors. Why do you need to change you Modicon processor to Siemens? Do you want your S7 to be your logic controller or simply a data concentrator?

I am not aware of any device that would allow an S7 device to talk directly to Modicon I/O.

One option is to install a gateway that allows the Modicon 785E to speak Siemen's protocols. One manufacturer is ProLinx at http://www.prolinxgateways.com/ they have Modbus and Modbus Plus capability. The logic in the Modicon stays where it is and your HMI thinks it is talking to a Siemens PLC.

Another option is to install an S7 processor that passes data back and forth to the Modicon. This method would require the S7 to have a Modbus or a Modbus Plus port. With this option, you can move the logic control to the S7 and have the Modicon just be an I/O servicer.
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Old December 26th, 2002, 01:25 PM   #3
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Thanks for the reply.

I need a Siemens PLC because the Siemens software package requires it. I am interested in a specialty version of Siemens PCS7 software. It does both SCADA controls on the HMI side and logic control for the PLC all in one.

I think your mention of using the S7 for control and the 785E as an I/O "gateway" is what I have heard before. Are there any concerns about doing this?
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Old December 27th, 2002, 10:50 AM   #4
Jim Dungar
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You need to consider what will happen to your I/O if there is a loss of communications.

One scenario:
S7 is reading the Modicon inputs, there probably won't be any problems, loss of communications = S7 sees all inputs as off.

S7 is writing to Modicon outputs, there is the probablity of dangerous operation, loss of communication = Modicon leaves all outs in last state.

You need some type of communications monitoring logic in the Modicon. This logic should turn off all of the outputs. Also, don't forget the Siemens needs to know that the communications link failed.
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Old December 27th, 2002, 01:28 PM   #5
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good point!

Thanks for the tip.
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