Advantages of using the Profinet IRT Gateway

[flk]

Hi,

I am wondering what the merits and demerits of these two setups are:

1. Using a RevPi Connect with Codesys license as Profinet device. No Profinet IRT Gateway.
   
   • merits: no Gateway needed, no need to transfer data (process image) between RevPi and Gateway (would it be faster/have less latency?).
   • demerits: Codesys license needed. Codesys does not seem to support Conformance Class C/IRT features.
   • unclear: What latency can I expect when the RevPi Codesys Profinet device receives a message from a Profinet controller and responds to it? As far as I know, Codesys is not providing IRT/conformance class C, but for option 2, the IRT part is still unclear to me.
2. Using a RevPi (e.g. Core S) with the Profinet IRT Gateway.
   
   • The RevPi copies the process image every ~5 ms. So while according to the data sheet, the Gateway supports Profinet IRT, I am really not sure where this IRT feature is beneficial . Does IRT mean that the Gateway and some (IRT capable) Profinet controller can exchange data quickly (and with a jitter <31.25 µs)? But what does that mean exactly and what is it useful for if I cannot access this data quickly enough? Through the RevPi, I can only read or write data to the Profinet with at least 5 ms delay (because of the process image update), right? So what is the benefit of using the IRT Gateway? In what kind of setup does the IRT part make a difference having a RevPi attached to the Gateway?

Thank you very much for any help,

Florian

[giuseppe]

Hello Florian,

unclear: What latency can I expect when the RevPi Codesys Profinet device receives a message from a Profinet controller and responds to it? As far as I know, Codesys is not providing IRT/conformance class C, but for option 2, the IRT part is still unclear to me.

The RevPi copies the process image every ~5 ms. So while according to the data sheet, the Gateway supports Profinet IRT, I am really not sure where this IRT feature is beneficial . Does IRT mean that the Gateway and some (IRT capable) Profinet controller can exchange data quickly (and with a jitter <31.25 µs)? But what does that mean exactly and what is it useful for if I cannot access this data quickly enough? Through the RevPi, I can only read or write data to the Profinet with at least 5 ms delay (because of the process image update), right? So what is the benefit of using the IRT Gateway? In what kind of setup does the IRT part make a difference having a RevPi attached to the Gateway?

On the subject of performance, it is difficult to give a direct answer in an open system as to how the performance of the respective device is. The performance like latency, cycle time,... is highly dependent on the customer application and what is being done with it.

I can only offer you the following article on performance.
RevolutionPi Hardware FAQ | Is there any information about the performance of the RevPi?

demerits: Codesys license needed. Codesys does not seem to support Conformance Class C/IRT features.

Please ask the Codesys support for this.
Bug reports or technical questions

So what is the benefit of using the IRT Gateway? In what kind of setup does the IRT part make a difference, having a RevPi attached to the Gateway?

The gateway is pre-configured and can be used "Plug and Play".
https://www.kunbus.com/profinet-irt-gateway-module

Here you can see what you can do with the Gateway modules.
https://kunbus-gmbh.atlassian.net/servi ... 2843508848

Best regards
Giuseppe Pagano

[flk]

Dear Giuseppe,

Thank you for your reply and the links. I am aware that the latency depends on the customer application, but I am more interested in some minimum latency which can be achieved in a minimal setup.

To be more clear with my questions:

1. What is the minimum latency which can be achieved when the RevPi with Profinet IRT gateway receives an Input (say a single Byte) and mirrors this Byte to the Output?
2. When is the IRT property of the Profinet IRT gateway useful? Is the IRT property required by some Profinet IO controllers? My current guess is that the communication between Profinet IO controller and the Profinet IRT gateway will be fast (and with very small jitter) but the data will still be accessible only after several milliseconds due to the delayed communication between Profinet IRT gateway and the RevPi itself?

Thanks for the helpful discussion

Florian


P. S.: As far as I understand, question 1 above corresponds to the duration of these steps:

• RevPi Profinet IRT Gateway receives profinet message
• RevPi gets Input from IRT Gateway
• RevPi mirrors Input to Output (will the use of mainloop() or cycleloop() be the better choice in terms of latency?)
• RevPi Profinet IRT Gateway receives the Output and sends out a Profinet message

[giuseppe]

Hello Florian,
As mentioned above, performance ratings are very difficult to answer.
Therefore, any statement must be taken with a grain of salt.

The gateways are initially designed for use as fieldbus couplers in a double pack:
Modular Gateway

Ideally, the cycle time between two gateway modules can be 1.2 ms - but this will certainly never be achieved in normal use.

There is no measurement with a Revolution Pi because it doesn't make as much sense there. This is because many factors can strongly influence the cycle time.

Each user must carry out a performance measurement himself. But there is no defined way how this can be done. Everyone has to decide individually how they want to do it.

Best regards
Giuseppe Pagano

[flk]

Dear Giuseppe,

Thank you very much for your reply

Kind regards,
Florian