Abstract: Data processing apparatus includes a number of units connected by a bus over which each unit can send public write messages to all the other units in parallel. The units are connected in a loop by means of public write acceptance lines. Whenever a unit receives a public write message it sends an acceptance signal to the next unit in the loop. Each unit produces an error signal if it receives a public write message but does ot receive any corresponding acceptance signal, or if it receives an acceptance signal without having received a corresponding public write message. Thus, each unit checks its neighbors in the loop to ensure correct reception of the messages.