RS232 doesnt use packets, so forget about that. Any packetising of data will be implemented by the software running on either side of the link.
CRC errors and bad bytes equally will need to be handled by the software either side.
Its really just a bit banging system. Feed a byte to a driver and it bangs it out as different voltage levels.
If you still have access to the two devices that are communicating then there is a method you can try to reverse engineer the "protocol" used between the units, allowing you to rebuild existing units or build new units as replacements or upgrades:
1. connect a PC with terminal software (e.g. PuTTY) to the controller and determine the baud rate. As you make changes to settings, or request readings, you will see said requests appear in the terminal software on the PC in some form, maybe simply as binary blobs or text based commands (e.g. AT commands when talking to a modem). It really depends on how the protocol is implemented.
2. connect the PC to the remote unit and issue the same commands and see what is returned
By doing this you learn what is done by the controller to make the other unit act or respond, and what should be sent back in response - if anything.
Without original source, or reference manuals to detail anything, this is about all you can do - short of disassembly. It could be a very long and arduous process depending on the complexity, or it may be very simple.