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. In larger systems multi-port serial IO boards are used, Digi International being one of most popular options. 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 allows 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/s, but in most system it is downgraded to 56.2 kbit/s or less to increase reliability.
RS-485 does not allow 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 and interfere 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 till 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 the regular Category 5 UTP network cable. Operation of the system is highly dependent on the host PC. In case the host PC fails, events from controllers are not retrieved and functions that required interaction between controllers (i.e. anti-passback) stop working.
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