Is there a way to expand the data frame of 1440 byte?
Posted: 23 Jul 2024, 11:08
Is there a way to expand the data frame of 1440 byte?
It has been understood for many years that the MTU size of an Ethernet frame is limited to 1500 bytes. Even if so-called jumbo frames are possible with the introduction of Gbit in some networks, these are not used by PROFINET. Jumbo frames are also a problem in long lines when Store&Forward switches are used. A 1500 byte long frame generates an S&F delay of approx. 125µs at 100 Mbit, at 8kb the value rises to around 650µs per switch!
To avoid having to extend the 1440 byte limit, the PROFINET specification allows a device to send more than one frame. However, it should be noted that the controller must also be able to send more than one frame to the device.
The question of why 1440 bytes per device are not sufficient should be much more decisive. A basic distinction can be made between values that must be transmitted cyclically and other values that can only be transmitted sporadically. And it is precisely for this sporadic communication that PROFINET has a service in "its belly" that every devkit supports: Data record communication. Up to 64 kb can be transferred consistently per data record. (It should be noted that the controller should also be able to read the amount of these data records. Most controllers support data records from 8 kB to 32 kB).
A simple example of data record communication: If I want to read out energy data, I am always interested in current and voltage per phase cyclically. I only need all other values such as apparent/reactive/active power every 15 minutes. Then I transfer these power values as data records, current and voltage in the cyclical data.