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Old March 17th, 2018, 12:52 AM   #1
Beau79AU
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Modbus Vs direct io on vfd

Hi all is there any hard and fast rule on using modbus vs direct io for controlling vfd's from a plc? I'm looking at using modbus to control 6 vfd from a plc to reduce the amount of io needed on the plc. Are there any big issues I need to worry about doing it this way?
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Beau
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Old March 17th, 2018, 06:37 AM   #2
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The main thing to consider is what happens if you lose communications between your PLC and the drive.

Most drives have the ability to detect when commands are no longer being written and can go to a safe state. Make sure this is enabled, and test that it works.
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Old March 17th, 2018, 11:22 AM   #3
Tom Jenkins
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There are few hard and fast rules about anything.

However, when I've used a comm link to control the drive I always had a hard-wired signal from my E-Stop and critical run permissive contacts. This would include things like lube oil pressure switches or a NC contact from the PLC. That protects the equipment even if the comm link goes down or is slow. You still have equipment protection.
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Old March 17th, 2018, 12:22 PM   #4
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Responsiveness is another consideration. How fast do your drives need to react to a change in the command? If the application can tolerate a couple of seconds delay from the VFDs then it can work. I do a lot of water pumping applications where it makes good sense to use Modbus RTU because it doesn't matter if the drives are a few seconds late changing speeds or starting or stopping.

Also, if at any time one or more of the drives is powered down, your PLC messages will error which will add a considerable amount of time to the polling cycle for those that are still up and ready.

All the VFDs I have worked with have a communication timeout action and delay. So you can program what you want the drive to do and how much time between polls is considered a communication loss.
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Old March 17th, 2018, 12:36 PM   #5
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I always used a timeout timer, the timer begins to count when I send the request and it's reset when I get the correct reply, if the time expires then I switch off via contactors the concerned equipment.
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Old March 17th, 2018, 12:51 PM   #6
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If Modbus is native to both PLC and Drives, then there isn't really a reason why not.

The PLC type and drive type would be of help though.
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Old March 17th, 2018, 04:06 PM   #7
rupej
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Only reason against modbus would be possible added complexity for the maintenance people that have to live with it, and harder to swap to a different model/brand of drive later on.

In other words, it makes it less "generic"
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Old March 17th, 2018, 07:24 PM   #8
Beau79AU
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Thanks for the replies. Will most likely be Schneider m221 plc and Schneider atv630 drives as we already have some of these drives. It is for running grout mixing barrels and pumps so a couple of seconds delay wont be too much drama.
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Beau
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Old March 18th, 2018, 12:44 AM   #9
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The M221 supports IOScanning of drives over Ethernet, if you have the model with Ethernet port.

Do a Google search for "M221 IOScanning", there's a video showing setup for an ATV320 but similar process for ATV630.

Much easier than rolling your own, given how clunky SoMachine is compared to Unity.
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Old March 18th, 2018, 04:24 AM   #10
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We generally start and stop the drive with digital I/O. All the rest is handled over the network whether it's ModbusRTU, ModbusTCP, or ModbusUDT. If your PLC supports ModbusUDT then use that. No reason to use ModbusTCP to communicate with a drive unless it's the only ethernet protocol it supports.

Good Luck,

Yosi
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Old March 18th, 2018, 02:23 PM   #11
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Quote:
Originally Posted by (8{)} ( .) View Post
We generally start and stop the drive with digital I/O. All the rest is handled over the network whether it's ModbusRTU, ModbusTCP, or ModbusUDT. If your PLC supports ModbusUDT then use that. No reason to use ModbusTCP to communicate with a drive unless it's the only ethernet protocol it supports. Yosi
I'm going to start a new topic because I see differences in RTU vs TCP for the hardware layer implementation and I'm curious as to who implements Modbus UDT and whether it is as 'reliable' as Modbus/TCP.
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Old March 18th, 2018, 02:51 PM   #12
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UDT or UDP ?
if it is really UDP then by definition it is less reliable than TCP, since using UDP the sender has no confirmation that the packet has really been received at the other end.
The advantage is that it can be faster than TCP.
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Old March 18th, 2018, 04:48 PM   #13
Pete.S.
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Quote:
Originally Posted by Beau79AU View Post
Hi all is there any hard and fast rule on using modbus vs direct io for controlling vfd's from a plc? I'm looking at using modbus to control 6 vfd from a plc to reduce the amount of io needed on the plc. Are there any big issues I need to worry about doing it this way?
Regards
Beau
Safety - as modbus is not a safety protocol so you can't transfer STO signals for instance.

Portability - you will be dependent on a specific model or brand VFD in your application.

Troubleshooting - if something is not working bus protocols are extremely hard to troubleshoot for electricians.
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Old March 19th, 2018, 01:18 AM   #14
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Quote:
Originally Posted by lfe View Post
UDT or UDP ?
if it is really UDP then by definition it is less reliable than TCP, since using UDP the sender has no confirmation that the packet has really been received at the other end.
The advantage is that it can be faster than TCP.
My typo...

Thanks for the correction.
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Old March 19th, 2018, 04:28 AM   #15
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One big advantage of modbus is that you will probably be able to grab a load of data from the VSD, not just control it. I don't know the Schneider drives so I don't know what data is available, in ABB you can get all the drive running data. Would it be helpful to know what power the drive is pulling, would it give you an idea of how the mixing process is going. We are no longer 'just' PLC programmers we also collect 'big data' to try to spot trends in machine data and improve efficiency.
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