24bit Standard Protocol

Can somebody recommend me a protocol to communicate with PIC USART with multibyte ex-24bit.It should have acknowledge frames.

Let say I have a PC & I have a master controller PIC & I have slave PICs (ex 6 slave PIC's).

I want to communicate with those three.Is there any standard protocol?

Not really, serial is only 8 bit (or seven bit), for 24 bit a packet system might be a good idea, but it really depends on the exact circumstances.