is used for low-speed data communications in commercial aircraft cabins' vehicle bus. It requires minimal wiring and can share the wiring among several seats, reducing weight. These are used in programmable logic controllers and on factory floors. RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. Utilizing a series of dedicated interface devices, it allows PCs and industrial controllers to communicate. Since it is differential, it resists electromagnetic interference from motors and welding equipment. In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. RS-485 serves as a physical layer for the AES3 digital audio interconnect. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. It may be used to control video surveillance systems or to interconnect security control panels and devices such as access control card readers. It is also used in Digital Command Control (DCC) for model railways. The external interface to the DCC command station is often RS-485 used by hand-held controllers or for controlling the layout in a networked PC environment. 8P8C modular connectors are used in this case. Protocols RS-485 does not define a communication protocol; merely an electrical interface. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. Interoperability of even similar devices from different manufacturers is not assured by compliance with the signal levels alone. Signals The RS-485 differential line consists of two signals: A, which is low for logic 1 and high for logic 0 and, B, which is high for logic 1 and low for logic 0. Because a mark (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage and space (logic 0) represented with a positive one, A may be considered the non-inverting signal and B as inverting. The RS-485 standard states (paraphrased): For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. For an on, space or logic 0 state, the driver's A terminal is positive relative to the B terminal. The truth tables of most popular