I am not aware of any site that allows downloading the protocol free of cost. ESTA & USITT are custodians for the show industry protocols. Being a member of ESTA's Control Protocols Working Group which governs DMX512, I would certainly discourage & report anyone who is offering DMX512A copy free for downloading.
A lot of hardwork by a lot of dedicated people has gone into inventing & formulating the protocol.Pl respect our efforts.
If you want purchase a copy, visit USITT at