readers referred to as "IP readers". Systems with IP readers usually do not have traditional control panels, and readers communicate directly to a PC that acts as a host. Some readers may have additional features such as an LCD and function buttons for data collection purposes (i.e. clock-in/clock-out events for attendance reports), camera/speaker/microphone for intercom, and smart card read/write support. Access control system topologies 1. Serial controllers. Controllers are connected to a host PC via a serial RS-485 communication line (or via 20mA current loop in some older systems). External RS-232/485 converters or internal RS-485 cards have to be installed, as standard PCs do not have RS-485 communication ports. Advantages: RS-485 standard allows long cable runs, up to 4000 feet (1200 m) Relatively short response time. The maximum number of devices on an RS-485 line is limited to 32, which means that the host can frequently request status updates from each device, and display events almost in real time. High reliability and security as the communication line is not shared with any other systems. Disadvantages: RS-485 does not allow Star-type wiring unless splitters are used RS-485 is not well suited for transferring large amounts of data (i.e. configuration and users). The highest possible throughput is 115.2 kbit/sec, but in most system it is downgraded to 56.2 kbit/sec, or less, to increase reliability. RS-485 does not allow the host PC to communicate with several controllers connected to the same port simultaneously. Therefore, in large systems, transfers of configuration, and users to controllers may take a very long time, interfering with normal operations. Controllers cannot initiate communication in case of an alarm. The host PC acts as a master on the RS-485 communication line, and controllers have to wait until they are polled. Special serial switches are required, in order to build a redundant host PC setup. Separate RS-485 lines have to be installed, instead of using an already existing network infrastructure. Cable that meets RS-485 standards is significantly more expensive than regular Category 5 UTP network cable. Operation of the system is highly dependent on the host PC. In the case that the host PC fails, events from controllers are not retrieved, and functions that require interaction between controllers (i.e. anti-passback) stop working. 2. 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 instead forward