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darmoll

ModbusTCP Speeds

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Hi, I'm currently looking into using ModbusTCP and I have some queries regarding transmission speeds. I have used ModbusRTU in the past, but have found it too slow for our current application. If we have 4 slave devices - 3 with baud rate of 100Mbs and 1 with baud rate of 10Mbs, is the overall network speed limited to the slowest transmission rate in the network (i.e. 10Mbs) or can the three 100Mbs devices still operate at the higher speeds? Is the communication between master/slave performed sequentially (as in ModbusRTU)? Also, say the 4 devices have a baud rate of 100Mbs and we're reading 100 16-bit words from each device, is the theoretical sample time for each word (4*100*16)/100e6 = 64 microseconds? How much would this differ in a practical test? I would appreciate it if anyone had some input to these questions, or can point me in the right direction. Thanks, Darren

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I doubt that you will be able to calculate speed on a TCP/IP network with any accuracy, too many variables. Looks good in theory and it should be much faster than serial but the data sent via MB-TCP/IP is miniscule compared to the overhead of Ethernet. Of course this needs to take into account any errors introduced or other traffic on the network. As to 10 or 100 that will be determined by the network, your selection and implementation of cabling, routers and switches. You will need to consult with those suppliers to get the best performance. Is it sequential or parallel? Again that is somewhat determined by the architecture of the Master device and how many "channels" it can handle, most will be 8,16, 32 or more. Plus what other traffic is on the network? If you have options and must have the fastest communications available today or a deterministic throughput look at Gigabit Profinet. Edited by RussB

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Many thanks for your response Russ. We are limited in our options. Our devices only support CanOpen and ModbusTCP, but our PLC (Mitsubishi Q-series) doesn't support CanOpen. I think ModbusTCP should be fine for our purposes. I imagine we will be limited by the scan time of our PLC anyway (~10ms, which is ok for us). I really just wanted to check that we weren't in danger of going slower than this

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CAN Open is not all that quick by the way - similar to RTU. Ethernet is quite slow - I would expect it to be slower than your PLC scan times. I have a job with 15 PLCs on Ethernet - all exchanging data. The refresh time is about 25ms on the Ethernet - and that is using UDP as well. When I go online with the PLC programming software and monitor - say - 10 PLCs, it is really pretty slow. Even a Token Ring network I use regularly has a refresh rate of 22ms - token ring is generally far more efficient than Ethernet.

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