Is it possible to get the "merged" data when I write a custom protocol analyzer?

This merging strategy is a part of the logging engine, so yes, it applies to all kinds of plugins, including custom protocol analyzers.

I concluded that the strange behaviour where I received data 1 byte each was because of an USB-UART adapter I used.

Hmm, a particular model of USB-to-UART is unlikely to cause this one-byte-at-a-time behavior. My guess is that the buffering settings are to blame (i.e., IO Ninja reads into a one-byte buffer). Check the "Buffering & compatibility" section in properties and try resetting it all to defaults.

thank you very much for your hint! I totally missed the check box "Highlight merged....". I concluded that the strange behaviour where I received data 1 byte each was because of an USB-UART adapter I used.

Just out of curiosity... Is it possible to get the "merged" data when I write a custom protocol analyzer?

By default, IO Ninja merges all blocks of the same data stream together (TX to TX, RX to RX) and highlights merged block boundaries using this grey-white checker pattern. You can turn off this highlighting and configure other details of the merging strategy here:

From your screenshot, I can see that (1) data arrives one byte at a time and (2) not all RX blocks are merged together.

(2) means that you modified the merging strategy (e.g. set to a 20ms threshold or something like that)

(1) most likely, it's caused by the custom buffering rules (e.g. the read block size is set to 1 byte). Unless you have a specific reason to do otherwise, it's recommended to use default buffer sizes.