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Is there really no link layer type that could be used for logging simple serial/COM port/UART communication?
No, because pcap/pcapng files are for use with traffic that can be divided into frames/packets at the lowest protocol level, but if all you're logging is bytes sent over a serial port, the only way to divide that into frames/packets would be to put each character sent over the serial port into a separate packet.
For half-duplex communication, it could, instead, accumulate all consecutive characters sent in a given direction into a single packet, or into multiple chunks with a maximum size (so you don't end up with a too-large packet, but no such link-layer type has been defined.
If a particular protocol sent over a serial port has framing, e.g. SLIP or PPP, then a link layer type could be defined for that particular protocol.
