EIA-485

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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.

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.

1 Uses of EIA-485
2 Connectors
3 Pin labelling
4 Waveform example
5 See also
6 External links

[edit] 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.

EIA-485 also is used to control theatrical and disco lighting where it is known as DMX.

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.

[edit] 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): [1]

RS-485 signal	RS-232 signal	DB-25	DB-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

RS-485 ISDN [2]	RS-485 signal (T1/E1 Telco)[3]	RS-485 ?[4]	(RS-232D EIA/TIA-561)[5]	RS-232 signal[6]	RJ-45 [7]
NC	RX+	TX1+	DSR(RI)	DTR	1 white/orange
NC	RX-	TX1-	DCD	DCD	2 orange
TX+	NC	RX2+	DTR	RTS	3 white/green
RX+	TX-	bidi3+	signal ground	RXD	4 blue
RX-	TX+	bidi3-	RXD	CTS	5 white/blue
TX-	NC	RX2-	TXD	TXD	6 green
NC	NC	bidi4+	CTS	Power(RI)	7 white/brown
NC	NC	bidi4-	RTS	Ground	8 brown
Ground	Ground			Ground	shield

[edit] Pin labelling

The RS485 differential line consists of two pins:

A aka '−' aka TxD-/RxD- aka inverting pin which is negative (compared to B) when the line is idle.
B aka '+' aka TxD+/RxD+ aka non-inverting pin which is positive (compared to A) when the line is idle.

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. [8] 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.