Query the Access Control Expert

Oct. 27, 2008
REX and Related Terms

Q: Can you explain the relationship between: REX, propped door, forced door, tailgating, anti-passback, piggy-back and muster report?

A: REX (Or Request to Exit) inputs on a door controller are used to trigger an unlock of the subject door, either for egress, for remote door control or sometimes for day/unlock modes. REX is used instead of directly controlling power to the locking device so that the features of the door controller are preserved such as lock reset, suppressing forced door alarms and providing an intelligible event on the audit trail. If the REX simply controlled power to the locking device, and the door control system was monitoring the position of the door, then opening the door without a valid access or REX would produce a forced door condition.

Many system designers will provide for both a signal closure on the REX input and also control the power to the locking device. This is advisable when the locking device is an electromagnetic lock and it is a life safety concern that the door unlocks in any situation.

Some REX circuits are one-shot triggers which mean that once the initial switch closure occurs across the REX terminals, the controller's internal timer will actuate and reset even if the short circuit is maintained across the REX terminals. In order to retrigger the controller it will be necessary to open the circuit across the REX to clear the input, and then close the circuit again.

Systems vary in this respect. You will need to evaluate how a particular piece of equipment operates before you can design how your door control will actually operate. There is the door open time and the propped door time. Sometimes you can program the unit to relock immediately after an access door opens, or as soon as it recloses after a valid access or REX, and the propped door timer will wait for the door position sensor to signal the door has reclosed.

If the door remains ajar, a propped door alarm will result. If your door control circuit does not permit these options you will be faced with having to program in a long door open time to accommodate the slowest system user. If someone follows behind under these conditions, it is referred to as tailgating. It is also known as piggy-backing but is more often used when referring to revolving door security access.

Unless you are controlling access and egress with a card reader, you usually don't care how many users egress with a REX trigger. In a revolving door, it can become a safety issue if more than one person occupies the system at a time. Revolving doors traditionally have a lower throughput than other type doors.

Anti-passback is a feature used to prevent the same credential being used for successive entries without egresses in between, or successive entries within too small a time window. Anti-passback is most effective when the premises is equipped with both entry and egress readers.

Keeping track of who is actually in a building is necessary to generate muster reports. Muster reports are of importance to rescue and firefighters so they know if there is someone actually inside a structure they need put themselves in harm's way to save.

Security Dealer Technical Editor Tim O'Leary is a 30-year veteran of the security industry and a 10-year contributor to the magazine. O'Leary's background encompasses having been a security consultant since 1986 and an independent security company owner/operator, in addition to his research and evaluation of new technologies and products introduced to the physical and electronic security fields. He is a member of the VBFAA (Virginia Burglar and Fire Alarm Association); certified for Electronic Security Technician and Sales by the VADCJS ( Virginia Department of Criminal Justice Services); and, has served as a judge for the SIA New Product Showcase. Send your integration questions to [email protected].