Connections used by "Generic modbus device"

[schnabber]

Hello,

Simple question that I cannot find the answer to.

If I have one generic modbus device configured (either via CPU or BMENOC0301) and need to read ~700 registers, I would then create 6 different "Requests" as the maximum registers per request is 125.

But how many concurrent connections would this open to the Modbus server, will there be 1 or 6 simultaneous connections to the server?

If someone have an link to somewere where exactly this is explained I would be greatful.

Also how the request and connection handling of the io scanner in general is of interest

Best regards.

[YONGHO-KIM]

1) I don't know what model the device is communicating with or what specifications it has from the slave's point of view, IP and port number are requests from the same master, so it is expected to be able to handle them.

2) If BMENOC0301 is also operated as a server, it can handle up to 64 units.

[schnabber]

Hi, and thanks for the reply

We are communicating to all kinds of devices, just depending on what different customers are using.

But usually you can find in their manuals that the Modbus server can handle eg. 8 simultaneous client connections via port 502.

So the main question is how many TCP connections the M580 will have open simultaneously towards any random (non Schneider) slave, when I have configured 1 "Generic Modbus Devices" which has 6 "Requests" configured.

What makes me wonder is when each request have their own "Connection bit", which could indicate they are each having their own TCP connection open towards the slave.

So I assume the answer is either 1, if the requests are being handled one at a time, or 6 if the requests are being polled all at the same time.

[YONGHO-KIM]

Based on the configuration you've described, if you have configured 1 "Generic Modbus Device" with 6 "Requests" and each request has its own "Connection bit," it indicates that each request can have its own TCP connection open towards the slave. Therefore, in this scenario, the M580 could have 6 TCP connections open simultaneously towards the non-Schneider slave, assuming that each request can be handled independently.
This means that if the requests are being polled simultaneously, the M580 could have 6 open TCP connections at the same time. I hope this clarifies the situation for you

[schnabber]

Thank you for your effort in replying. I do really appreciate it
But you answer implify that you are not sure about the actual situation, and it then only becomse a reformulation of my original question.
What I would need to know is if the polls are actually being performed simultaneously or one at a time.
A better way to ask the same thing is maybe: will the polls have different source ports?
I do not have any CPU available for the moment, but I guess I will have to check it with wireshark as soon as I get my hands on the hardware.

[YONGHO-KIM]

I share the test result value using Modbus Simulator.

1. Test conditions
1) Poll Delay 1s
2) Poll Delay 0.1s

2. Poll Delay 1s
- Interval of the register requesting 125 pieces approximately 2 ms

3. Poll Delay 0.1s
- Interval of the register requesting 125 pieces approximately 2 ms

[schnabber · Accepted solution]

Here is the answer if someone else is wondering the same as I did.

According to what I can see, each configured request will open its own TCP connection towards the modbus server.

From the wireshark logg you can see that as soon as the modbus server is started, the M580 establishes 2 separate TCP connection [SYN, ACK] with source ports 3456 and 4737 against the modbus server on 502.

Same source ports are being used (same TCP connections kept open) until the server is stopped [FIN, ACK].

Attached are 3 pictures:

1st: Request configuration

2nd: Wireshark screenshot from when Modbus server is started

3rd: Wireshark screenshot from when Modbus server is stopped

[schnabber · Accepted solution]

Here is the answer if someone else is wondering the same as I did.

According to what I can see, each configured request will open its own TCP connection towards the modbus server.

From the wireshark logg you can see that as soon as the modbus server is started, the M580 establishes 2 separate TCP connection [SYN, ACK] with source ports 3456 and 4737 against the modbus server on 502.

Same source ports are being used (same TCP connections kept open) until the server is stopped [FIN, ACK].

[schnabber · Accepted solution]

Attached are 3 pictures:

1st: Request configuration

2nd: Wireshark screenshot from when Modbus server is started

3rd: Wireshark screenshot from when Modbus server is stopped

[YONGHO-KIM]

1st: Request configuration
> If it is possible to share PLC project file, I will review it.

[Trinxs1]

Schnabber and Yongho-Kim the answer is that for the DTM instance created for one device with multiple requests there will be one connection for each request even though the same IP address is targeted.  This has been confirmed as you have done by Wireshark and by the collective intelligence in the global technical presales team.  

[MatthewM]

Just for fun, can you get onto the webpage for the CPU or BMENOC0301, whichever is being used, and look at the connection list there?  It may list the same IP twice, indicating the two connections.  I am sure I have seen it before somewhere.

[schnabber]

I can do so when i get my hands on an CPU again, might be next week. Would be good if the webpage indicated not only active connections but all connections that's configured (trying to connect).