With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method. Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). Ideally, the two ends of the cable will have a termination resistor connected across the two wires. The equipment located along a set of RS-485 wires are interchangeably called nodes, stations or devices. This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, rs485 cable the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered. Termination resistors also reduce electrical noise sensitivity due to the lower impedance.

The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. It also defines three generator interface points (signal lines); A, B and C. The data is transmitted on A and B. C is a ground reference. Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. RS-485 only specifies the electrical characteristics of the generator and the receiver: the physical layer. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. The issue also exists in programmable logic controller applications. These are used in programmable logic controllers and on factory floors. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals. If A is negative with respect to B, the state is binary 1. The reversed polarity (A positive with respect to B) is binary 0. The standard does not assign any logic function to the two states. For an on, space or logic 0 state, the driver's A terminal is positive relative to the B terminal.

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. However this contradicts the facts that an idle mark state is a logical one and the termination polarization puts B at a higher voltage in Profibus guidelines. For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. RS-485 is used for low-speed data communications in commercial aircraft cabins' vehicle bus. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. RS-485 and RS-422 can interoperate with certain restrictions. RS-422 and RS-485 Standards Overview and System Configurations, Application Report (pdf). TSB-89A, Application Guidelines for TIA/EIA-485-A does not recommend using star topology. The foreword to the standard references The Telecommunications Systems Bulletin TSB-89 which contains application guidelines, including data signaling rate vs.

These characteristics make RS-485 useful in industrial control systems and similar applications. It is also used in Digital Command Control (DCC) for model railways. In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. Electrical signaling is balanced, and multipoint systems are supported. It has a special capacitive sensor designed by us, efficient and of good precision, waterproof, resistant to oxidation and not subject to variations due to salinity and soil composition: better than this there are only the "extraction pressure" sensors (which we do not propose because they are not durable and also the sensor alone costs hundreds of euros). Not only that, it can also be used to know if there is water in any place or not; for example, positioned appropriately, it can be used to know if a tank or a drinking trough is full or empty, or if a well has reached a certain level (eg to stop pumping water). This robustness is the main reason why RS-485 is well suited for long-distance networking in noisy environment. PC environment. 8P8C modular connectors are used in this case.