RS-485
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. Electrical signaling is balanced, and multipoint systems are supported. The standard is jointly published by the Telecommunications Industry Association and Electronic Industries Alliance (TIA/EIA). Digital communications networks implementing the standard can be used effectively over long distances and in electrically noisy environments. Multiple receivers may be connected to such a network in a linear, multidrop bus. These characteristics make RS-485 useful in industrial control systems and similar applications.
Standard[edit]
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. 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.[6] The initial edition of EIA RS-485 was dated April 1983.[7]
RS-485 only specifies the electrical characteristics of the generator and the receiver: the physical layer. It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. The foreword to the standard references The Telecommunications Systems Bulletin TSB-89 which contains application guidelines, including data signaling rate vs. cable length, stub length, and configurations.
Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. These characteristics include: definition of a unit load, voltage ranges, open-circuit voltages, thresholds, and transient tolerance. 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. 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.
$_$_$DEEZ_NUTS#0__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#0__subtitleDEEZ_NUTS$_$_$
Full duplex operation[edit]
RS-485, like RS-422, can be made full-duplex by using four wires.[8] Since RS-485 is a multi-point specification, however, this is not necessary or desirable in many cases. RS-485 and RS-422 can interoperate with certain restrictions.[9]
$_$_$DEEZ_NUTS#6__descriptionDEEZ_NUTS$_$_$
Converters, repeaters and star topology[edit]
Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. By using repeaters very large RS-485 networks can be formed. TSB-89A, Application Guidelines for TIA/EIA-485-A does not recommend using star topology.[10]
$_$_$DEEZ_NUTS#2__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#2__descriptionDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#3__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#3__descriptionDEEZ_NUTS$_$_$
Protocols[edit]
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.
$_$_$DEEZ_NUTS#5__heading--0DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#5__description--0DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#5__heading--1DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#5__description--1DEEZ_NUTS$_$_$
The RS-485 differential line consists of two signals:
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):[14]
The truth tables of most popular devices, starting with the SN75176, show the output signals inverted. This is in accordance with the A/B naming used by most differential transceiver manufacturers, including:
These manufacturers all agree on the meaning of the standard, and their practice is in widespread use. The issue also exists in programmable logic controller applications.[c] Care must be taken when using A/B naming. Alternate nomenclature is often used to avoid confusion surrounding the A/B naming:
RS-485 standard conformant drivers provide a differential output of a minimum 1.5 V across a 54-Ω load,
whereas standard conformant receivers detect a differential input down to 200 mV. The two values provide
a sufficient margin for a reliable data transmission even under severe signal degradation across the cable
and connectors. This robustness is the main reason why RS-485 is well suited for long-distance
networking in noisy environment.[28]
In addition to the A and B connections, an optional, third connection may be present (the TIA standard requires the presence of a common return path between all circuit grounds along the balanced line for proper operation)[29] called SC, G or reference, the common signal reference ground used by the receiver to measure the A and B voltages. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. The allowable common-mode voltage is in the range −7 V to +12 V, i.e. ±7 V on top of the 0–5 V signal range. Failure to stay within this range will result in, at best, signal corruption, and, at worst, damage to connected devices.
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. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents – enough to melt signal cables, PCB traces, and transceiver devices.
RS-485 does not specify any connector or pinout. Circuits may be terminated on screw terminals, D-subminiature connectors, or other types of connectors.
The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds.
$_$_$DEEZ_NUTS#7__heading--0DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#7__description--0DEEZ_NUTS$_$_$
Applications[edit]
RS-485 signals are used in a wide range of computer and automation systems. 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. RS-485 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. DH 485 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.[11] 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[12] or for controlling the layout in a networked PC environment. 8P8C modular connectors are used in this case.[13]
$_$_$DEEZ_NUTS#5__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#5__descriptionDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#6__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#7__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#7__descriptionDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#4__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#4__subtextDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#9__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#9__subtextDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#8__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#8__subtextDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__titleDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__subtextDEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--0DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--1DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--2DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--3DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--4DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--5DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--6DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--7DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--8DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--9DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--10DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--11DEEZ_NUTS$_$_$
$_$_$DEEZ_NUTS#1__answer--12DEEZ_NUTS$_$_$