In clean room areas,any personnel passing through an unclassified area to a classified area is required to pass through a series of rooms. In each room, there are certain steps that need to be followed. This procedure ensures that the person is not carrying any external contamination into the process area and also the pressure in each room is maintained before opening the door. Door Interlock Controllers achieve this by preventing simultaneous access of doors by ensuring that when one door of a room is open then the other door is closed. Push buttons are available for door access. Emergency stations are available for opening of all doors. There is a delay between each door opening to maintain required pressure. Door status can be read by any master system connected over the RS485 network. This door interlocking system shall interface with the fire alarm system