TIA-485-A, also known as ANSI/TIA/EIA-485, TIA/EIA-485, EIA-485 or RS-485, is a standard defining the electrical characteristics of drivers and receivers for use in balanced digital multipoint systems. Digital communications networks implementing the EIA-485 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, multi-drop configuration. These characteristics make such networks useful in industrial environments and similar applications.
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. The RS-485 standard is superseded by TIA-485, but often engineers and applications guides continue to use the RS designation.
RS-485 enables the configuration of inexpensive local networks and multidrop communications links. It offers data transmission speeds of 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 RS-422), it can span relatively large distances up to 1,200 m (4,000 ft). A rule of thumb is that the speed in bit/s multiplied by the length in meters should not exceed 108. Thus a 50 meter cable should not signal faster than 2 Mbit/s.
What is an RS-485 network?
RS-485 allows multiple devices (up to 32) to communicate at half-duplex on a single pair of wires, plus a ground wire (more on that later), at distances up to 1200 meters (4000 feet). Both the length of the network and the number of nodes can easily be extended using a variety of repeater products on the market.
How does the hardware work?
Data is transmitted differentially on two wires twisted together, referred to as a “twisted pair.” The properties of differential signals provide high noise immunity and long distance capabilities. A 485 network can be configured two ways, “two-wire” or “four-wire.” In a “two-wire” network the transmitter and receiver of each device are connected to a twisted pair. “Four-wire” networks have one master port with the transmitter connected to each of the “slave” receivers on one twisted pair. The “slave” transmitters are all connected to the “master” receiver on a second twisted pair. In either configuration, devices are addressable, allowing each node to be communicated to independently. Only one device can drive the line at a time, so drivers must be put into a high-impedance mode (tri-state) when they are not in use. Some RS-485 hardware handles this automatically. In other cases, the 485 device software must use a control line to handle the driver. (If your 485 device is controlled through an RS-232 serial port, this is typically done with the RTS handshake line.) A consequence of tri-stating the drivers is a delay between the end of a transmission and when the driver is tri-stated. This turn-around delay is an important part of a two-wire network because during that time no other transmissions can occur (not the case in a four-wire configuration). An ideal delay is the length of one character at the current baud rate (i.e. 1 ms at 9600 baud). The device manufacturer should be able to supply information on the delay for their products.
Two-wire or four-wire?
Two-wire 485 networks have the advantage of lower wiring costs and the ability for nodes to talk amongst themselves. On the downside, two-wire mode is limited to half-duplex and requires attention to turn-around delay. Four-wire networks allow full-duplex operation, but are limited to master-slave situations (i.e. a “master” node requests information from individual “slave” nodes). “Slave” nodes cannot communicate with each other. Remember when ordering your cable, “two-wire” is really two wires + ground, and “four-wire” is really four wires + ground.
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 RS-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 itself would provide termination if it were 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, to maximize signal strength and therefore line distance and speed. Applying the bias at multiple node locations could possibly cause a violation of the RS-485 specification and cause communications to malfunction.
The RS-485 differential line consists of two pins:
- A aka ‘+’ aka Data + (D+) aka TxD+/RxD+ aka non-inverting pin
- B aka ‘-‘ aka Data – (D-) aka TxD-/RxD- aka inverting pin
- SC aka G aka reference pin.
The SC line is the optional voltage reference connection. This is the reference potential used by the transceiver to measure the A and B voltages.
The B line is positive (compared to A) when the line is idle (i.e., data is 1).
In addition to the A and B connections, the EIA standard also specifies a third interconnection point called C, which is the common signal reference ground.
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.
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. These are used in programmable logic controllers and on factory floors. 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 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 model railway: the layout is controlled by a command station using Digital Command Control (DCC). The external interface to the DCC command station is often RS-485 used by hand-held controllers or for controlling the layout in a network/PC environment. Connectors in this case are 8P8C / RJ45.
Although many applications use RS-485 signal levels; the speed, format, and protocol of the data transmission is not specified by RS-485. Interoperability of even similar devices from different manufacturers is not assured by compliance with the signal levels alone.