The abort control circuit was an important component of the transporter's function. On Constitution-class starships, the circuit was located with the main transporter circuits on the lower level of the engineering deck.
In 2266, the circuit on the Enterprise was completely destroyed due to a phaser blast, necessitating emergency repairs. This meant there would be further delays to retrieving Hikaru Sulu and the rest of the landing party on Alfa 177. (TOS: "The Enemy Within")