Whether the transmission is parallel or serial, you wish to packetise the data. The general method is:

header (9th bit set or special sequence)
payload (the data)
check (checksum or crc)

One method I tried was the header has the destination address with the 9th bit set then followed by the packet length (this can be two bytes for large packets)
the data then follows and appended to that is your check byte (or word).

THe packet length is required as this tells you where the end of the data is unless you have some other means of terminating your packet.

I do question your use of fifo's. If it is a distributed controller - do you really need the high speed? Most plc's use async serial to talk to each other. Tell us more about your application and constraints.