I found Dynamixel tutorial on protocol very clear and newbie friendly: Dynamixel Protocol User Guide - Robotis
https://emanual.robotis.com/docs/en/dxl/protocol1/

I  am now using a modified form of Dynamixel for my python programming as summarized below:(1) Controller (eg. Arduino, Rpi) and serial bus servo (eg. AX12A, LX-224HV) communicate each other by sending and receiving data called Packet. 

(2) There are two kinds of packets: 

(a) Instruction (Control) Packet, which Main Controller sends to control (request status/data of) servo, and 

(b) Status (Return/Reply) Packet, which servo replies/responses to Controller.