Master-Slave master side SW architecture with Custom RS422 full duplex protocol

[GuyB 0]

Hey Guys,
i'm working on implementing the master side of a custom serial master-slave protocol between a B&R PLC and an electronic card.
As part of the data i'm sending and receiving is used to close various loops(e.g. cascaded temperature control where the temperature sensors and actuators are connected to the electronic card) , but other data is not as usefull (e.g. various data used for display or at specific times), i believe i will also need to implement some sort of priority mechanism.

my first question is - based on your experience - how would the general master side SW architecture look like? 
(for example - the send side of mine is comprised of 3 tasks all running under the same cycle - lower level task in charge of opening the port, sending the data etc, a mid level task in charge of creating the packet based on command from upper level, giving commands to lower level and using status from it to continue and finally - an upper level task in charge of deciding which messages to send).

my second question is - how would you implement the priority mechanism?

thanks