There are a lot of good suggestions and ideas here and notice that there are different ways to tackle it for different purposes. Having used many serial protocols both good and bad as well having made several of my own (both good and bad...) here are a few of my suggestions and comments.
Keep it simple. I have found the
greatest success with simple header
based command-response "packets".
Don't worry about human readable ASCII. It is only useful for the few hours you actually debug your protocol. After that it is restrictive to always encode data as ASCII and very inefficient if you transfer a lot of data.
Use error checking. I prefer a 16 bit CRC as it provides orders of magnitude of protection over a checksum and is still simple and efficient over heavier algorithms like MD5 or SHA1.
Use the same packet format for the commands as the responses.
Use 8 bit data with no parity. Serial parity bit doesn't add anyprotection if you already use a CRCor other data integrity check and isa poor error check at best.
So in the simplest form the packet header is the Command or Response, size of the packet, zero or more dataand the error checking code (CRC).
