I have a CMM321 talking Modbus TCP to 5 remote nodes (Beckhoff). The problem is that with a single node enabled, I can read/write in about 0.25 seconds. Not fast but acceptable. If I enable a second node, the speeds drops to 0.4 second per node. With all five nodes, it can be between 1 and 1.5 seconds. Way too slow.

I did extensive testing and troubleshooting.
I am now sure that the bottleneck is the CMM321 card. It seems to have a very limited bandwith and shares the bandwith with all active connections. Up to 16 channels can be configured. I'd hate to see the scan time on those.
My rep is helping me but I would like to know what kind of scan time other users are getting with Modbus TCP.

If you have a similar setup and it communicates faster, please let me know how you did it.

 

I've managed about 80% of theoretical between two Linux PLCs, but you'll never get that if you send one packet per scan with a 10 mS scan time. And most of the IO I've seen can't handle back to back packets or anything like full speed 10mhz. Ethernet rates. But from my actual tests, GE's "foreign" protocol support is calculated to drive you to the "native" protocols. On Profibus, I couldn't get much better than 50 _hz_! So you should be content that it works. If you need Ethernet bandwidth, you should consider using something that can fill the pipe. I don't see any common PLC that could do that by their nature and the way they write to the network. Perhaps someone has seen differently?

Regards

cww

 

I have gotten speeds less than 1ms to multiple I/O devices or drives using Modbus/TCP. I used Ethereal to measure speeds using a Modicon Quantum scanning I/O or drives. If can be very fast..