75. SERIAL MAIN & SUB COTROLLERS

Serial main and sub-controllers. All door hardware is connected to sub-controllers (a.k.a. door controllers or door interfaces). Sub-controllers usually do not make access decisions, and forward all requests to the main controllers. Main controllers usually support from 16 to 32 sub-controllers. Advantages:Work load on the host PC is significantly reduced, because it only needs to communicate with a few main controllers.


The overall cost of the system is lower, as sub-controllers are usually simple and inexpensive devices. All other advantages listed in the first paragraph apply.
Disadvantages:Operation of the system is highly dependent on main controllers. In case one of the main controllers fails, events from its sub-controllers are not retrieved and functions that require interaction between sub controllers (i.e. anti-passback) stop working.


Some models of sub-controllers (usually lower cost) have no memory and processing power to make access decisions independently. If the main controller fails, sub-controllers change to degraded mode in which doors are either completely locked or unlocked and no events are recorded. Such sub-controllers should be avoided or used only in areas that do not require high security. Main controllers tend to be expensive, therefore such topology is not very well suited for systems with multiple remote locations that have only a few doors.


All other RS-485-related disadvantages listed in the first paragraph apply. In such case the intruder would gain access only to the communication cable going from the reader the input/output module and would not be able to unlock the door by shorting or cutting the wires. Possibilities of manipulating the input/output module via the communication cable are very low as in most cases proprietary protocols are used, the data is encrypted and modules are configured to communicate only with their host readers.