Multiple Masters on a Modbus TCP / IP Network

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Thread Starter

Cam Lowry

In a system with one PLC providing process control and monitoring a series of pump VFD's and process water chillers, we wish a second PLC to be "backup". If the main PLC fails, the second PLC would take over operation. is this possible to have two master PLC's on a MODBUS TCP/IP network?

During normal operation, the main PLC (IDEC)read / writes to the pump VFD's and chiller Microprocessors. The backup PLC is actually the customers DCS that runs the plant. We would like the DCS PLC to also read from the VFD's and chillers simultaneously. In the event of a fault in the main PLC, the DCS would then have the ability to take over operation of the system.

This setup would require there are two master controllers on the MODBUS system. Is this possible? All devices (IDEC PLC, DCS, VFD's, Chillers)on the system are homerun to an Ethernet IP hub.

Thanks for your advice!

Cam
 
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Hi Cam,

Of course you can on Modbus TCP/IP, but I think it is not the same for Modbus RTU. I suggest you to establish the both tcp connections on the start of the system to avoid the connection delay time in case of fault of the first PLC.
 
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I have been down this path - your challenge is not having two modbus masters on the network, this is natively supported.

The problems you will face are;

* Do the VFD / Modbus Slaves perform well enough (at a network level) to accept two Modbus Masters - some devices while supporting on paper - actually don't have a robust implementation of network stack / modbus handling - i.e.; beware the device that is actually Modbus RTU with a "cheerful" ethernet port shoved in front.

* Are your PLC and DCS systems flexible enough to play nicely / negotiate over which device is controlling the slaves?

* Can you disable Modbus writes from PLC or DCS at will (not all systems support this)?

* Can you configure your slave devices to accept two sets or writes and determine which set they will use?

* Can you tolerate / maintain the level of complexity this will add to your design? What about the next person to come after you?

I suggest you consider your overall design - and criticality of control / failure modes etc. Maybe hardwired control of critical drives is better for you, and use Modbus TCP as read only for "bulk" data transfer.

PB
 
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Walter Secrest

Yes, it is possible to have all devices (IDEC PLC, DCS, VFD's, Chillers) on the system are homerun to an Ethernet IP hub.

If the VFDs and Chillers have ethernet ports then you won't have to make any hardware changes. Just configure the DCS, main PLC and backup PLC to ensure only one is writing to these devices at any time - I suggest adding PLC or DCS option buttons to your HMI.
If the VFDs and Chillers have Modbus or Modbus Plus ports only then add "Modbus TCP/IP to Modbus RTU converter". Configure the main PLC, backup PLC and DCS to communicate with the chiller and VFD using ethernet only. The converter will become the only master on the Modbus / MBP network. This converter will give you alot of flexibility by allowing many PLC, DCS, SCADA, PC, etc to access these devices. If you want to speed up communication or want to isolate sections of your Modbus / MBP then simply install additional converters. The ethernet network will allow simultaneous read (and write) to these devices. Again, add option buttons to the HMI to select the PLC or DCS as the only device permitted to write.
 
Hmm, it seems to me that your problem raises more questions rather than just "how to get 2 modbus masters on one link"

What exactly is the PLC doing to control the pumps? Does it have sequencing, duty/standby, HMI functions etc?
What exactly is the DCS going to do if the PLC fails? Does the customer still expect to have the same level of functionality in the DCS? Does this mean you will have to write DCS software to duplicate PLC software
How critical is this PLC that you want to backup? Would redundancy be a better solution?

Which is more reliable, the PLC or the DCS? can you just ditch the PLC?

While yes, it is certainly possible to have 2 masters on a Modbus TCP network, it is possible you may run into other more practical problems about how to switch control from one system to another.

Rob
www[.]lymac.co.nz
 
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N. Kariniemi

@Cam : 2 Modbus TCP Masters
If you have Modbus slaves, whether they be serial Modbus RTU, serial Modbus ASCII, or Modbus TCP, the Devicemaster UP from Comtrol will allow you to connect multiple masters. 96 Modbus TCP Masters, in fact. I've never pushed it that high (who WOULD!!?) but I have run a mix of different serial and ethernet based masters to a mix of slaves where I had 65 Masters polling 50+ slaves and it worked great. (This was done with PLC emulators for experimental purposes, since I am the guy who tests this gateway.) You can load different firmware builds into the hardware depending on your needs. Sounds like a good fit for you would be "Modbus Router."

http://www.comtrol.com/pub/en/DeviceMaster-UP-Modbus-Solutions

Give us a call at 1-763-494-4100 (8am to 6pm M-F) if you have questions.
 
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