Abstract: The bases of modular units of an I/O system are connected serially by two-way electrical and mechanical connectors so that units can be inserted into and removed from lines of units connected to a controller without removal or movement of any other unit. By two-way connector is meant that confronting connectors on adjacent units disengage by movement in the same or opposite direction from that in which they engaged. The electrical connectors distribute a communication bus and power to the modular units. In addition, plug-in electronics modules and personality modules, with the latter providing selective interfaces between the electronics modules and banks of terminals for landing field wiring, have a common locking mechanism for locking both in sockets in the bases. The locking mechanism includes at least one locking assembly having a shaft rotatable from a free end of the electronics module for rotating a paddle into and out of engagement with a catch in the base.