RS 485 is a widely used communication interface in security access control and video surveillance. RS485 is an OSI Model physical layer 1 electrical specification of a two-wire, half-duplex, multipoint serial connection. The standard specifies a differential form of signaling. The difference between the two wires’ voltages is what conveys the data rather than an absolute voltage level between either wire or ground.
As an example, when we measure voltage to an electric lock, we measure between positive and ground, and we are looking for a specific steady reading, for example 24 VDC. With RS485 we measure between the two wires, usually identified as A and B. There may be a shield present but we do not measure between the shield, and either A or B. By the way, the RS485 voltage will fluctuate, and probably the reading with be unsteady and changing too rapidly for your meter to actually track a level. Because the RS485 does not use ground for a reference it prevents ground loops from being a source of communication problems and makes RS485 useful for connections between buildings etc, or where there won’t be a common ground shared by the equipment.
RS 485 is used in proprietary system networks which interconnect devices such as access controllers, and pan/tilt/zoom control on video cameras. Examples of access control systems that use RS485 loops are: CANSEC; Millennium Enterprise, and Ingersoll Rand Security Technologies Security Management System.
Twisted pair cable with a shield is usually specified for RS485. But when running a cable is not convenient, or economical or possible, other alternatives exist.
I recently faced just such a dilemma when we contracted to add a door to an existing access control system. The existing system included three different buildings. Each building had a controller to which individual door interfaces were connected via RS485. Each controller connected to the LAN, and was administered by a server.
Each controller could manage up to 16 reader interfaces.
Administration and monitoring of the system was performed at workstations with hierarchal privileges assigned to individual users as required for them to manage their building or department, or to monitor activity on doors within their department or management responsibilities.
Our new door was going to be in a new building where only one access door was planned. So this one door would require a RS485 connection between the door interface and one of the controllers which had an open channel. There were open channels in each controller, but establishing a RS485 connection from our new reader interface to any of the controllers presented a problem, since the buildings were at separated by distance, roadways, sidewalks and utility lines.
We had already installed an aerial cable between two of the other buildings, but in that application, there were no intervening utility lines or roadways. There was a sidewalk, and that was what prevented us from trenching so we ran a cable about 20 feet up from building.
The issues in planning this current deployment were the length of cable that would be required; the height and consequential dangers required to transgress obstacles and keep the cable out of the way; and the equipment costs for the messenger cable and hardware; the articulated boom which would have to be leased. Additional labor would be required to run cable vertically down to the ground floor ceilings where controls and access control hardware were located, and horizontal runs above ceilings to where the access control hardware was located. The work would have to be performed while the premises, a private school campus, was in operation.
For the previous shorter run, we improvised, using outdoor rated cable attached to a galvanized guy wire.
For this longer run, we felt a special messenger cable would be required, and this material was not readily available, meaning an extended lead time, and a required minimum purchase which would greatly exceed our requirements. In this case we could not pass along the expense of the excess materials to the customer. We would need to purchase 1000 feet of exotic messenger cable, use about 150 feet of it, and have to store the rest hoping we would have a future use for it, which was not likely, so it would be a loss.
A reader, keypad or combination of both is an essential ingredient in virtually any access control system, and in the majority of systems the reader will be a prox reader. Although biometric...
The Zodiac Lite Kit comes with a single reader, door control module, and hand-held programmer. No cards, PC, or software are required.