IEC 61076-2-010 specifies circular connectors with a push-pull locking mechanism, including both internal and external locking systems. Initially, Phoenix Contact released M12 push-pull with external locking connectors, also known as “click-lock”. The connector springs back if not mated properly and was developed with railway applications in mind. It is important to note, M12 push-pull with external locking for railway applications is not backward compatible with standard M12 connectors, unlike the internal locking version.