EIA-485, also known as RS-485, is a digital interface standard for wired, differential, serial data transmission. Due to the symmetrical signal transmission EIA-485 is characterized by a high tolerance to electromagnetic interference.
EIA-485 uses a pair of lines, to transfer a non-inverted and the inverted level of a 1-bit data signal. The receiver is reconstructed from the difference between the two voltage levels, the original data signal. This has the advantage that do not affect the common mode disturbances on the transmission and thus the interference is increased. In contrast to EIA- 232 is much longer transmission distances and higher clock rates are possible. Compared to the EIA- 422 standard have the transmitter by an integrated resistor short circuit proof output stages, so that a counter- ends of two channels does not lead to defects. On a pair also several transmitters and several receivers may be connected to (' MultiPoint ').
In contrast to other buses only the electrical interface conditions are defined in EIA-485. The protocol can be selected for specific applications. Therefore, be EIA -485 devices from different applications or manufacturer generally involves. do not understand. If data is to be transported, often one uses the character transmission of Universal Asynchronous Receiver Transmitter Protocol ( UART), known by RS- 232 interfaces. Most here eight equivalent bits per frame.
The name derives from the Electronic Industries Alliance ( EIA), which published its standards formerly called RS for Radio Sector. With the expansion of subject areas over the radio technology addition, although the prefix was retained, but was officially no longer abbreviation. Today RS is usually read as a Recommended Standard. The standard is managed now by the Telecommunications Industry Association ( TIA short ), has the designation TIA -485 -A and the title of Electrical Characteristics of the generator and receiver for Use in Balanced Digital Multipoint Systems ( ANSI/TIA/EIA-485-A- 98 ) ( R2003 ). The last part of the name indicates that the standard in 2003, with no technical changes was confirmed.
The two symmetrical lines of the EIA- 485 interface to work with a differential voltage level of at least / -200 mV. The sender of a typical 485 - block uses a bridge circuit, thus the signal level corresponds to the sender of the operating voltage of the driver, such as / - 5 V. In contrast to the mass-based EIA- 232 interface or to the old TTY current loop interface of the Telegraph by the symmetrical structure of the signal conductor 485 a receiver against common mode noise largely insensitive.
It uses only one pair in the rule and is operated in half duplex, with two pairs but also full-duplex operation is possible. The connection is multi-point capable which means it can store up to 32 stations ( or unit loads, a hypothetical load size) are connected to the EIA- 485 bus. There transceiver modules, which are 1/2, 1/4 or even 1/8 Unit Load. These blocks are then compared to a normal transceiver a double, four times or eight times the input resistance. For networks with up to 256 participants can be established ( 1/8 Unit Load ). Usually, cable lengths up to 1.2 km and transfer rates up to 10 Mbit / s are supported, and the maximum transfer rate is achieved only with cable lengths up to 12 m. The actual possible maximum network size and the maximum transfer rate is also strongly dependent on the structure of the network. In particular, star topologies should be avoided because of the resulting long stubs advantageous is the structure in the form of chain ( daisy chain ).
Since the EIA- 485 interface is a bus system (as opposed to point-to -point connection with EIA- 232), the cable ends should be completed (at least for longer cable lengths or higher rates ). In general, a passive termination, by connecting the signal lines is used for each of a 120 Ω resistor to the two ends of the bus. An optional bias network avoids undefined bus level for inactive line drivers. Otherwise, the receiver remains in undefined bus in the state of the last active logic value, but can tilt by strong noise level back and forth.
Long lines may be caused by the voltage drop to larger potential differences between the bus nodes that interfere with communication. This can be improved by carrying the ground line or avoided by galvanic isolation ( optocouplers).
EIA- 485 only specifies the electrical characteristics of the interface, it does not define a protocol and no pin assignment. Therefore, there is no uniform pinout of EIA- 485 connector, so that when using different EIA- 485 devices, the documentation of the device must always be consulted. When Profibus, based on the EIA -485 standard, pins 3 and 8 used from 9- pin D- Sub connectors and sockets for the data line, for example.
Wide dissemination has EIA- 485 also in the checkout area, where joined by IBM printers and other peripherals through proprietary connectors.
The DMX interface used in event technology for light control is based on EIA- 485th
Overview of the main specifications
The non -inverted line is usually identified with A or or .. P (positive), the inverse of B, - or N. .. In two buses would be the example And - or RXTX RXTX -P and -N, at 4 bus lines according to TX , TX -, RX , RX-. Mass is, for example, SG Signal Ground.
EIA -485, EIA-422
Because of its similarity to EIA -485, EIA- 422, are often used interchangeably, EIA -422 is considered a subset of the EIA-485 standards. However, both standards are not 100% compatible with each other. EIA-485 components may be used without any problems, although in the EIA-422 networks, but not vice versa. Important differences are:
- EIA -422 devices usually have no Driver Enable, which the transmitter can be switched off. In a network with multiple drivers EIA-422 devices can not be used is because it can happen otherwise, that two drivers drive towards each other. EIA -422 devices are not protected against it.
- For EIA -422 components that have yet via a Driver Enable, the drive strength is not enough to drive a terminated at both ends of the network.
- The common-mode voltage resistance is not as big as in the EIA-485 receivers in EIA-422 drivers. If multiple drivers with common-mode voltages and offsets occur in networks that are not allowed for the receiver, the maximum allowable voltage at the driver may already be exceeded.
In EIA -485 EIA-422 network components should therefore only be used as a receiver. However, it must still be noted that an EIA- 422 receivers with his 4 kOhm input impedance is equivalent to 3 Unit Loads.
The now-deprecated interface EIA-/RS-232 ( point-to- point connection), in contrast to the newer standards EIA-/RS-422 (a station several recipients ) and EIA-/RS-485 ( multiple transmitters, multiple recipients ) other level conditions. During the RS- 232 is only designed nearly pseudo- differential ( signal reference for all signals in both directions GND) is EIA- 485 designed clearly differentially. In order to tolerate the intermodulation between the various signals are bidirectional RS -232, RS- 232 were increased in stroke and thresholds. Corresponding Interface ICs for RS -232 (eg MAX232 ) realize this, an additional integrated auxiliary power generation ( eg / - 12 volts).
With the newer interfaces ( EIA-422, EIA -485) is recognized, however, that the quality of a balanced signal transmission is completely independent of the height of the switching threshold (center potential). Just Taking the switching threshold midway between VDD and GND ( 5 volts to 0 volts) is achieved with ordinary BiCMOS blocks one of the EIA- 232 superior transmission quality.
While the minimal form of RS- 232 from a transmission signal TX, a receive signal RX and a ground (GND) can exist, RS -485 comes with two signals A and B, working together in the push-pull once in one direction and second time in the other direction transport data. The resulting risk of the counter - sending two partners due to a slightly higher circuit complexity.
This disadvantage are facing serious advantages. On the one hand does not apply to RS-422/-485 the generation of positive and negative supply voltages down completely, so the components effort is reduced considerably, on the other hand allow the now smaller signal swings a significant reduction in power dissipation, as an adaptation to the ( independent of the signal amplitude ) impedance of the line is always required is. As a side effect also occurs with the same displacement current () a gain in speed, with a shorter stroke and the same slope of the level has reached the final value faster. Thus, the data transfer rate and distance are significantly increased despite reduced power dissipation.
Since opposite line ends must also be avoided in two point connections via communication protocol already implies RS- 485 actually the ability to form multi-point networks without circuitry overhead.