Maximum Profinet devices Motors

Question: Can I put as many profinet drives as I want onto my profinet ring and if so how.

I have a project where I will be controlling a large number of drives; 90+
I will be controlling the start/rev/running/fault from digital I/O but I would like to write setup parameters and speed control over profinet.

Lets take the example that I use a Siemens S7 1500 top spec and for drive I use Danfoss FC302 with MCA120 profinet cards added, off the top of my head I think the PLC stats state 64 profinet devices.

I believe there are two methods of profinet comms. One method is a cyclic update and I think I am limited to the number of profinet devices mentioned in the PLC stats. Another method is periodic update.

Is it possible to set any device to do a periodic update over profinet and therefore have a virtually unlimited number of devices and if so how.

The numbers I've seen for MRP are something along the lines of a max recovery time of 200ms for a max 50 devices. I don't think I've seen a ring larger than that, and I don't think I'd recommend it.

The first reason is from a strict networking perspective. Every switch the data passes through has a little bit of delay (potentially 10-15us), and after 50 switches, that means your delay is up to half a millisecond. In real world terms that is really short, but that can be massive in the drives/motion world. In networking terms, that's certainly a large delay. When your packet delay starts getting longer than the reconfiguration time of the network, you start risking the ring manager thinking there is an error in a functional network.

The second reason is from a usage perspective. The ring allows the network to survive one error. If there are two errors, which could be as simple as a drive powered down for maintenance, no data gets to any devices between the two faults.

Honestly, rings are great as a backup mechanism, but I don't recommend a ring larger than 10 or so devices, regardless what the specs say are possible.

That doesn't mean, however, that I would recommend limiting your PN network to 10 devices. A top end 1500 (1518) can control 700+ PN devices out of its built in port, and more if you add comm cards. You should use switches and a star type topology to get the data between the PLC and the drives.

64 node limit sounds like Profibus.

Firstly thanks for the reply.

I should have chosen my words more carefully.
I won't have a single ring, I will be doing a Tree network; star->Ring.

My longest length of devices will be 30 or less depending on Panel layout.

I am going to have 5 remote I/O stations via profinet on this system each of them will be star to the switch.

But basically you are saying I can have as many of these drive devices on the profinet network as long as I create a stable network and they will not effect the response time of my remote I/O.

By adding additional comms cards will this in any way negatively effect the system like a slower cycle time, If I was to add some they would all be on the same Network and subnet; would this cause any issues?

In the Profibus world, the entire profibus network was a shared medium. That means only one device, whether a PLC or an IO node, could talk at a time. That means there had to be limits on how many nodes in a segment, and if you added a 2nd Maser, the bandwidth got cut in half.

By moving to Ethernet, you use a switched medium. Each device talks to its parter, and then the partner forwards the data along, as it needs to. This means greatly increased speeds, because everyone can talk at the same time. In addition, the link bandwidth goes up, so that you have 100Mbps, instead of maxing out at 12Mbps.

In addition, PN traffic is Unicast, which means that once the network is running, data only travels on the path that it needs to get from the source to the destination. Multicast or broadcast traffic goes everywhere, which uses up much more bandwidth. Traffic from one device only affects another device if they happen to travel through the same cable.

Because every device is independent, you can also set independent cycle times/update rates for different devices on the network. This means a few devices might need to be updated quickly, and the rest could be much slower.

There isn't really a defined limit to the number of PN devices that can be on the same Ethernet network, or the number of Controllers on the same network. There reaches a point where there is enough broadcast traffic that the IT guys start being uncomfortable, but that is more likely defined by the rest of the network traffic, not the PN. Every PN controller has its limit, but there is nothing built in as part of the standard. In portal, if you dig into the Ethernet settings of the PN controller interface, it should tell you how much of the theoretical limit your PN traffic should be. I've rarely seen a system that went over 10%.