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EIA-485 (formerly RS-485 or RS485) is an OSI Model physical layer electrical specification of a two-wire, half-duplex, 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 networks 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 metres).
In contrast to EIA-422, which has a single driver circuit which cannot be switched off, EIA-485 drives 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.
The recommended arrangement of the wires is as a connected series of point-to-point 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 and two powered resistors to bias the lines apart when the lines are not being driven. 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 but bias resistors are required. The value of each termination resistor should be equal to the cable impedance (typically, 120 ohms for twisted pairs). Without biasing resistors, the signal falls to zero (where electrical noise sensitivity is greatest) when all stations are silent or unpowered. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance (all those resistors! or none!).
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.
Uses of EIA-485
- SCSI-2 and SCSI-3 (for instance) use this specification to implement the physical layer.
- EIA-485 is often used with common UARTs to implement low-speed data communications in commercial aircraft cabins. For example, some passenger control units use it. It requires minimal wiring, and can share the wiring among several seats. It therefore reduces the system weight.
- EIA-485 also sees some use in programmable logic controllers and on factory floors in order to implement proprietary data communications. Since it is differential, it resists electromagnetic interference from motors and welding equipment.
- EIA-485 is used in large sound systems, as found at music events and theatre productions, for remotely controlling high-end sound-processing equipment from a standard computer running special software. The EIA-485 link is typically implemented over standard XLR cables more usually used for microphones, and so can be run between stage and control desk without laying special cables.
- EIA-485 also is used in Building automation as the simple bus wiring and long cable length is ideal for joining remote devices.
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): *
| Signal | RS-232 signal | DB-25 | DE-9 | RJ-50 |
|---|---|---|---|---|
| Common Ground | Carrier Detect (DCD) | 8 | 1 | 10 |
| Clear To Send + (CTS+) | Received Data (RD) | 3 | 2 | 9 |
| Ready To Send + (RTS+) | Transmitted Data (TD) | 2 | 3 | 8 |
| Received Data + (RxD+) | Data Terminal Ready (DTR) | 20 | 4 | 7 |
| Received Data - (RxD-) | Common Ground | 7 | 5 | 6 |
| Clear To Send - (CTS-) | Data Set Ready (DSR) | 6 | 6 | 5 |
| Ready To Send - (RTS-) | Request To Send (RTS) | 4 | 7 | 4 |
| Transmitted Data + (TxD+) | Clear To Send (CTS) | 5 | 8 | 3 |
| Transmitted Data - (TxD-) | Ring Indicator (RI) | 22 | 9 | 2 |
Waveform example
The graph below shows potentials of the two wires of an RS-485 line during transmission of an RS-232 byte:
Wire A (non-inverting) = RxD+ / TxD+
Wire B (inverting) = RxD- / TxD-
External links
- Guidelines for Proper Wiring of an RS-485 (TIA/EIA-485-A) Network
- RS232 to RS485 cable scheme
- supplier for RS232 to RS485 converter
- Advantech Isolated RS-232 to RS-422/485 Converter
"EIA-485"