How much data can be exchanged using a PN/PN Coupler in a profinet network?

I am using PCS7 V9.1 SP2.

On some sources i read 256 bytes input and 256 output.

And other manuals say 988 bytes.

So i am confused and asking for your guidance. :)

In PCS7 using the Siemens PN coupler library and S2 as base requirements, then I'd agree 988 is max.

The 254/244 bytes is referring to the max per HW module slot on the coupler if using a PN/MF coupler and there is a warning to watch out that its possible to exceed the CFC IO limits.

If you are not using Siemens library for PN/PN coupler, its possible to exchange more data, but I don't think that's what you are asking about.  At that point its all custom code and adapting TIA Portal configuration limits to PCS7 programming, theoretically up to 4096 using data records or 1440 using cyclic only which would need to include the coupler overhead.

By the way, why do we need the PN PN Library? Isnt it enough just to use APL Challel blocks like AI FB1813 or AI for field devices FB1869?

And how can i read from a pn pn coupler acyclically?

PN/PN coupler is not covered by the “area of application” of the APL channel driver blocks ( you can check this in PUD).  You need to treat the PN/PN coupler as a generic PN device and build your own IO mapping, or buy the library.  If you try to use them, you will have compilation warnings as if you used them for other non-supported devices.