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This helps to minimize the impact of electromagnetic interference, causing the voltage between the two wires to deviate. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. The EIA has officially disbanded and the standard is now maintained by the TIA as TIA-485, but engineers and applications guides continue to use the RS-485 designation. RS-485 does not define a communication protocol; merely an electrical interface. Rs485 cable is essentially a signal cable with a twisted pair structure, additionally shielded with 2 layers of noise protection used in industrial communication. It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. RS-422 and RS-485 Standards Overview and System Configurations, Application Report (pdf). The EIA once labeled all its standards with the prefix "RS" (Recommended Standard), but the EIA-TIA officially replaced "RS" with "EIA/TIA" to help identify the origin of its standards.

Alarm systems: Rs485 cables in alarm systems help transfer alarm signals and information from alarm devices to the control center. These characteristics make RS-485 useful in industrial control systems and similar applications. The issue also exists in programmable logic controller applications. 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. A (green, negative) is higher than B (red, positive). For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. 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. 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. For an on, space or logic 0 state, the driver's A terminal is positive relative to the B terminal.

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. Care must be taken when using A/B naming. Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together - it is wise to add some current limiting to the SC connection. Digital communications networks implementing the standard can be used effectively over long distances and in electrically noisy environments. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. Since RS-485 is a multi-point specification, however, this is not necessary or desirable in many cases. RS-485 serves as a physical layer for the AES3 digital audio interconnect. 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. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans.

TSB-89A, Application Guidelines for TIA/EIA-485-A does not recommend using star topology. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. What is rs485 cable? Rs485 cables not only vary in the structure of the core but also in the number of twisted pairs. Twisted pair structure: Rs485 anti-interference cable has a twisted pair conductor structure. Control and monitoring systems: A single pair of rs485 signal cable can connect up to 32 devices, with a signal transmission distance of up to 1200m. Therefore, using rs485 cables is an ideal choice for industrial control and monitoring systems, especially those with large-scale and high dispersion. 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. RS-485 only specifies the electrical characteristics of the generator and the receiver: the physical layer. RS-485, also known as TIA-485(-A) or EIA-485, is a standard, originally introduced in 1983, defining the electrical characteristics of drivers and receivers for use in serial communications systems.