July 4, 2013

What is RS485 ?

The need for RS485 –

With the advent of Industrial Revolution, more & more factories started churning out goods for the mass market. This necessitated high quality & fast machinery for production. For this to happen it became necessary to have a coordinated effort of all the machinery concerned. And thus a special branch of Factory Automation started to emerge.

For factory automation, it is necessary to collect all the information at a point where critical decisions are to be made. This required a robust communication channel which can survive industrial environment & yet provide reliable communication between many nodes – points. Keeping these requirement in mind, an international standard was agreed upon & was published by Telecommunication Industry Association/ Electronic Industry Alliance (TIA/EIA). This is known as TIA 485. Though TIA earlier used “RS” ( Recommended Standard) prefix for its standards and so this standard is much more popular as RS485 rather than TIA485. We will also refer this standard as RS485 only.

RS485 Standard –

This standard ( RS-485-A or ISO/IEC 8284) defines the electrical characteristics of interconnection including driver, line & receiver. It allows data range up to 35 Mbps ( Million Bits per Second) and the lengths of up to 1200 meters. Though both of these can not be achieved at the same time. That is to say you can not have RS485 network with the length of 1200 meters & having 35 Mbps data rate.

Speed –

The speed of 35 Mbps can be achieved with lengths of up to 10 meters & reduces to 100 Kbps for lengths up to 1200 meters. Generally it is observed that the speed in Bits per second multiplied by length in meters should not exceed 108. So if your network is 100 meters long then the data rate will be around 1 Mbps maximum. The RS485 standard only specifies only electrical characteristics but it does not specify or recommend any communication protocol for this network.

RS485  Network –

RS485 network uses twisted pair of wires to carry differential signals. The data is transmitted along this twisted pair of wires generally referred as “A” & “B”. Ground reference is referred as “C”. Section 4 of the standard defines electrical characteristics of the system like Unit Load ( UL ), Voltage Range, Open circuit Voltages, various thresholds etc. The logic state – data – of 1 & 0 is defined by the polarity of A & B terminals. If A is negative with respect to B then the state is considered as Binary 1 while opposite condition where B is negative with respect to A is considered as 0.

RS485 network consists of 2 or more transceivers or at least one transmitter & one receiver. This enables the configuration of inexpensive local networks and multidrop communication links. The recommended connection of wires is to have series of nodes connected to it point to point way just like a line or bus. This should not be a ring or star multiple connected network.

Ideal condition is to have Master node at the middle of network which is extended both ways. The both ends should have termination resistors connected between two wires. Without termination resistors, the data gets corrupted due to reflections of fast driver edges. These resistors also reduce electrical noise sensitivity due to lower impedance. Some times bias resistors along the network are also used to make network ” Fail Safe”. By connecting pull up & pull down resistors, a defined voltage levels are provided. This helps nodes not to interpret the noise from undriven lines as actual data.

Though RS485 is generally employes “Half Duplex” communication that is only one device can transmit data at a time over the network, it can be made ” Full-Duplex” by using 4 wires.

Applications –

Because of easy wiring & robust network characteristics of RS485, it has been extensively used in Factory & Industrial Automation systems. Many standard industrial protocols like Interbus-S,Modbus & Profibus are based on RS485 as a physical layer. It is also used in many Aircraft s. Lately is is also used in Home & Building Automation because of its simple wiring & long cable length availability. Many remote devices can be connected to the Network & monitored easily.

The availability of RS485 to other format converters provides an easy solution for controlling & monitoring the network by PC. Format converters like RS485 to RS232 and USB to RS485 converters help to connect many remote devices to PC.

We have also RS485 based Relay cards that help you not only to control low level devices but also Electrical devices of 230 AC. You can have your own Home Automation System using these RS485 based Relay and RS485 based DAQ cards.


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