This article will go over how HMIs are typically used in a PROFINET context. Often, the decision on which protocol to use on an HMI depends on your application requirements and what protocol drivers the HMI supports. The main differences between TCP/IP-protocol based HMIs and PROFINET-based HMIs are the following:
Most HMIs are not time critical
An HMI is for use by a human operator to interface to a machine. In typical applications this is not time critical as most HMIs monitoring the process will update in some seconds or hundreds of milliseconds and the data packets used are typically larger. Usually, there is also a lot of data being exchanged, so TCP/IP also makes sense to be able and send thousands of bytes. The HMI operator will not notice a difference between a second or 500ms when viewing some HMI screens or changing values.
PROFINET can make your HMI real time
However, sometimes you might need some real-time control from an HMI and more precision for certain applications. For example, in the case of an HMI helping control the process with push buttons, e-stop or to issue certain time-critical commands (real-time operator input, LED output) this requires real-time control and a protocol like PROFINET. Most of the time, the PROFINET messages are small because we are typically talking bits and bytes with the IO devices to turn something on or off. In the event we want the HMI to communicate PROFINET, it would act as an IO device and send inputs to the controller application in real time as well as receive outputs from the controller.
HMIs can support multiple protocols
Some HMIs can also speak multiple protocols. If PROFINET is supported, you could opt to have nonreal-time (TCP) and real-time (PROFINET) communication in your HMI simultaneously. That is a big advantage of using HMIs with PROFINET due to its added real-time (RT) functionality. It should be stated that PROFINET is also an open Industrial Ethernet network where you can have HMIs, PROFINET IO devices, and standard TCP/IP based devices on the same network without issues. As TCP based HMIs can add a lot of traffic, be sure to check the PROFINET design guidelines when designing your system if you have some of those.
Editor’s note: This article is an excerpt from PROFINETuniversity.com