Chaining 23 Profibus MM430 Drives to CPU

[John Gaunt]

I have a customer who wants a major upgrade to a system I installed in 2000. It will be mostly a start again job with new panels but reusing some equipment. There will be 23 PID Cooling control loops that are to be MM430 Drives connected to a S7 CPU using Profibus. Can I chain all these drives to just one profibus port. I plan to use CPU 6ES7-414-2XK05-0AB0 but this is not yet final. I plan to use the other CPU port for communications to Citect SCADA. The system will also have 63 other PID controllers using a mix of Increase Decrease Digital Outputs to Electric Actuators and 4-20 mA outputs to Pneumatic Actuators.

I understand there is a limit to the number of Profibus devices but don't have any details.

 

[Peter Nachtwey]

You should be OK. You need a repeater for every 32 nodes. One network can have 126 nodes though.

 

[John Gaunt]

Thanks Peter, that seems to answer my first concern.

I am also concerned about the CPU resources required for 23 Profibus drives. In the past I have used Increase Decrease Digital Outputs with Analogue feedback to control drives but have no experience using Profibus for this purpose.

 

[John Gaunt]

Also wondering if there is a Siemens FB for handling the drives via Profibus. Or can sombody point me in the right direction regarding programming for the drives.

 

[rohan tandon]

Siemens has a sample project for communicating with micromaster drives over profibus.

http://support.automation.siemens.com/WW/view/en/22078757

I have been following your posts since i used to work in the glass industry, it appears like a refiner / forehearth conditioning system that you are upgrading... I would recommend a separate 24v power to the profibus comm modules on the drives and an 'active profibus termination' for the network.

 

[John Gaunt]

Thanks Rohan,
Yes it will be Distributor and 4 Forehearths once re-built.
I have downloaded the Siemens example and am working through it.
Regarding your comments about a separate power supply and Profibus termination:
I assumed the profibus power would come from the PLC CPU module Profibus port. Where would I connect a separate supply?
The only Profibus termination I have come across is the switch in the Siemens Profibus plugs. What other termination do you recommend?

 

[KalleOlsen]

Hi John!

Here's an active terminator from Procentec. They also have repeaters and hubs, take a look.

http://www.procentec.com/products/productdisplay.php?SIG=920052

Kalle

 

[John Gaunt]

Thanks KalleOlsen,

I guess for me the Profibus active terminator is like looking at a solution when I have no understanding of the problem it solves. I notice the recommended unit has two 24 VDC supply ports. I guess this is the separate 24 VDC supply that rohan tandon refered too. Again I don't know why I shouldn't simply rely on the PLC CPU module to provide Profibus power.

I hope somebody can explain in more detail.

It has also been suggested to me that I should limit the number of Drives to 10 before using a repeater. Again, I don't understand this as other advice says 32 units is OK. Is the repeater a Siemens product?

I need to provide a highly reliable system without any manual intervention. I know I can do this using ABB drives controlled by Increase Decrease Digital outputs with an analogue speed feedback. Hopefully I can provide an equivalent system with MM drives and Profibus.

 

[rohan tandon]

Thanks KalleOlsen,

I guess for me the Profibus active terminator is like looking at a solution when I have no understanding of the problem it solves. I notice the recommended unit has two 24 VDC supply ports. I guess this is the separate 24 VDC supply that rohan tandon refered too. Again I don't know why I shouldn't simply rely on the PLC CPU module to provide Profibus power.

I hope somebody can explain in more detail.

It has also been suggested to me that I should limit the number of Drives to 10 before using a repeater. Again, I don't understand this as other advice says 32 units is OK. Is the repeater a Siemens product?

I need to provide a highly reliable system without any manual intervention. I know I can do this using ABB drives controlled by Increase Decrease Digital outputs with an analogue speed feedback. Hopefully I can provide an equivalent system with MM drives and Profibus.

Hi john,

I will try and explain why i did recommend active termination and the 24v dc power supply for the comm. modules...


The profibus option module for the MM430 drives has a little orange connector near the profibus connector, this is to provide a 24V power supply to power the profibus module separately. The advantage is the profibus communication between the PLC and the Drive is not lost when the drive is powered down.

The active termination unit is an alternate to the inbuilt terminator on profibus plugs. Termination of the profibus nodes is really critical on both ends. If there is a problem with the profibus comm module on the last drive on the network then you may have network fault on other nodes as well, due to improper termination as well as a faulty drive / comm module. I have seen this problem many times, an active termination module is inexpensive and highly recommended.

Regarding the repeater, it is usually used if the segment length is more than a certain distance or the number of nodes are more than 32. The length of a segment @ 1.5mbps is about 200 metres without repeaters. You can comfortably use 23 nodes without a repeater.

 

[KalleOlsen]

Hi again, John!

Your Profibus runs on RS485, and this standard requires a powered end terminal resistor network.

It has to do with the signal powering and with suppression of cable end reflections.

Besides end termination there are several other things to have in mind. The most important is good earthing and good equipotential between the cabinets. The profi cable must be treated well and not be laid close to power cables, especially carefully with VSDs.

I'd recommend you to spend some time with the installation guidelines, the link is here:
http://www.profibus.com/nc/downloads/downloads/profibus-installation-guideline/display/

Edit: Scroll to the page bottom.

Kalle

 

[John Gaunt]

Thankyou KalleOlsen and rohan tandon for your assistance.
I will do some more reading and try to understand more.

I am rather concerned by the possible problem in introducing another power supply that might not be tied to the PLC CPU module. I assume this needs to be considered and the 0VDC side of the additional supply be tied down to Electrical Ground. I assume the CPU has its DC tied to Electrical Ground.

I note the concern with wiring practices. I will ensure that this is done properly. In fact in the particular plant concerned I have in the past had to threaten to get back on a plane in order to get an earth connected.

 

[iant]

Check your Private message
Give Hercule my regards

 

[John Gaunt]

Thanks iant, I will talk with him.

I have 2 other Profibus devices on the system being 2 PC's used both for Step 7 programming and as Citect SCADA. I am considering these being on the same Profibus network as the 23 MM430 Drives. Is this OK or should the PC's be on a separate Profibus network.

If I configure both of the CPU ports as Profibus I assume on the rare occaisions I might need to use a PC Adaptor I can use it via Profibus rather than MPI as I do now.

 

[KalleOlsen]

Thanks iant, I will talk with him.

I have 2 other Profibus devices on the system being 2 PC's used both for Step 7 programming and as Citect SCADA. I am considering these being on the same Profibus network as the 23 MM430 Drives. Is this OK or should the PC's be on a separate Profibus network.

If I configure both of the CPU ports as Profibus I assume on the rare occaisions I might need to use a PC Adaptor I can use it via Profibus rather than MPI as I do now.

You can put all devices on the same line, but it will slow down the bus cycle considerably. Because you have 3 masters on the same bus, and each master has its time slot.

Do you know which protocol the 2 PCs uses?
It is most likely not DP (raw I/O), rather S7. In that case, you should put them on a separate bus and connect them to the MPI slot.

Kalle

 

[John Gaunt]

Thanks KalleOlsen,

The 2 PC's each have a Siemens CP5613A2 Profibus card installed. The Citect Driver is known as S7NT.

If I connect the 2 PC's to the MPI port:
1. I assume I should reconfigure that port as Profibus.
2. If I need at any time to use the PC adaptor can I do that using Profibus?

I will connect the MM drives to the other Profibus port.