Information AboutEia-485 |
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EIA-485 (formerly '''RS-485''' or '''RS485''') is an , Multipoint Serial Connection . The standard specifies a differential form of signalling. The difference between the wires’ voltages is what conveys the data. One polarity of voltage indicates a logic 1 level, the reverse polarity indicates logic 0. The difference of potential must be at least 0.2 volts for valid operation, but any applied voltages between +12 V and -7 volts will allow correct operation of the receiver. EIA-485 only specifies electrical characteristics of the driver and the receiver. It does not specify or recommend any Data Protocol . EIA-485 enables the Configuration of inexpensive Local Network s and multidrop communications links. It offers high Data Transmission speeds (35 Mbit/s up to 10 m and 100 Kbit/s at 1200 m). Since it uses a Differential Balanced Line over Twisted Pair (like EIA-422 ), it can span relatively large distances (up to 4000 Feet or just over 1200 Metre s). In contrast to EIA-422 , which has a single Driver Circuit which cannot be switched off, EIA-485 drivers need to be put in Transmit mode explicitly by asserting a Signal to the driver. This allows EIA-485 to implement linear Topologies using only two lines and between the ribbon cable. The equipment located along a set of EIA-485 wires are interchangeably called nodes, stations and devices. The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, a line or bus, not a star, ring, or multiply-connected network. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to the lower impedance, and bias resistors (see below) are required. The value of each termination resistor should be equal to the cable impedance (typically, 120 ohms for twisted pairs). Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. Somewhere along the set of wires, powered resistors are established to bias each data line/wire when the lines are not being driven by any device. 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, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered. Often in a master-slave arrangement when one device dubbed "the master" initiates all communication activity, the master device itself provides the bias and not the slave devices. In this configuration, the master device is typically centrally located along the set of EIA-485 wires, so it would be two slave devices located at the physical end of the wires that would provide the termination. The master device would provide termination if it itself was located at a physical end of the wires, but that is often a bad design as the master would be better located at a halfway point between the slave devices. Note that it is not a good idea to apply the bias at multiple node locations, because, by doing so, the effective bias resistance is lowered, which could possibly cause a violation of the EIA-485 specification and cause communications to malfunction. By keeping the biasing with the master, slave device design is simplified and this situation is avoided. EIA-485, like EIA-422 can be made Full-duplex by using four wires, however, since EIA-485 is a multi-point specification, this is not necessary in many cases. EIA-485 and EIA-422 can interoperate with certain restrictions. RS-485 can be used to communicate with remote devices at distances up to 4000 ft (1200 m) at speeds of up to 100 kbit/s at this distance. Converters between RS232 and RS485, USB and RS485, Ethernet and RS485 are available to allow your PC to communicate with remote devices. By using "Repeaters" and "Multi-Repeaters" very large RS485 networks can be formed. The Application Guidelines for TIA/EIA-485-A has one diagram called "Star Configuration. Not recommended." Using an RS485 "Multi-Repeater" can allow for "Star Configurations" with "Home Runs" (or multi-drop) connections ''similar'' to Ethernet Hub/Star implementations (with greater distances). Hub/Star systems (with "Multi-Repeaters") allow for very maintainable systems, without violating any of the RS485 specifications. Repeaters can also be used to extend the distance and/or number of nodes on a network. It can and is being done. USES OF EIA-485
This standard is now administered by the TIA and is titled TIA-485-A, ''Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems (ANSI/TIA/EIA-485-A-98) (R2003)'', indicating that the standard was re-affirmed without technical changes in 2003. CONNECTORS EIA-485 does not specify any connector. The following table lists some typical RS-485 signal pin assignments ( RS-232 , another serial standard, listed here for comparison): {Link without Title} Note that the correspondence between RS-485 and RS-232 signals applies only to the pins of the DB-25 connector, not the DA-9 or RJ-50 connectors. PIN LABELLING The RS485 differential line consists of two pins:
These names are all in use on various equipment, but the actual standard released by EIA only uses the names A and '''B'''. However, despite the unambiguous standard there is much confusion about which is which: The RS485 signalling specification states that signal A is the '''inverting''' or '-' pin and signal '''B''' is the '''non-inverting''' or '+' pin. {Link without Title} The same naming is specified in the NMEA standards. This is in conflict with the A/B naming used by a number of differential transceivers manufacturers, including the Texas Instruments application handbook on RS422/485 communications (A=non-inverting, B=inverting). These manufacturers are incorrect, but their practice is in a widespread use. Therefore, care must be taken when using A/B naming. In addition to the A and '''B''' connections, the EIA standard also specifies a third interconnection point called '''C''', which is the common ground. WAVEFORM EXAMPLE The Graph below shows Potentials of the '+' and '−' pins of an RS-485 line during Transmission of an RS-485 Byte : REFERENCES SEE ALSO EXTERNAL LINKS
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