RS485 Communication

RS485 Communication was developed to provide high speed data. The standard is defined by the telecommunication bodies of the sector and can be more commonly referred to as RS485 Communication, but references to EIA485 or TIA-485 can also be seen.

RS485 Communication is capable of providing a 10 Mbps title data rate at distances of up to 50 feet, but the distances can be extended to 4000 feet with a slower speed of 100 kbps.

Although RS485 was never designed for home use, it has found many applications where remote data acquisition was required.

RS-485 connections are often used for simple networks and can be connected in 2 or 4 wire mode. In a typical application, multiple devices that support addresses can be connected to a single controlled PC (PC), and therefore a single line can be used for communication. It is also possible to convert between RS485 and RS232 using simple interface converters which can include optical isolation between the two circuits, as well as the suppression of overvoltages for any electrical “spikes” that can be detected.

Using RS-485, it is possible to build a multipoint data communication network. The standard specifies that up to 32 controllers or transmitters can be used together with 32 receivers in a system. This means that there can be 32 nodes capable of transmitting and receiving. This can be further expanded by using “automatic” repeaters and high impedance receivers / controllers. In this way it is possible to have hundreds of nodes in a network. In addition, RS485 extends the common mode range for both tri-state controllers and receivers with the power turned off. In addition, RS-485 controllers can support “data collision” (bus containment) problems and bus failure conditions.

As the RS485 Communication networks get bigger, the problem of data collisions increases. This can be solved, at least in part, by ensuring that the hardware units (converters, repeaters, microprocessor controls) are designed to remain in reception mode until they are ready for data transmission.

Another approach is to design a “single teacher” system. Here the master initiates a communication request to a “slave node” addressed to that unit. The hardware detects the transmission start bit and then enables the transmitter. Once the requested data is sent, the hardware goes back to receiving mode.

RS485 specification overview

The table below provides the highlight details behind RS485.

RS485 HIGHLIGHT SPECIFICATIONS
ATTRIBUTESPECIFICATION
CablingMulti-drop
Number of devices32 transmitters
32 receivers
Communications modeshalf duplex
Maximum distance4000 feet @ 100 kbps
Maximum data rate10 Mbps @ 50 feet
SignallingBalanced
Mark (data = 1)
condition
1.5 V to 5 V (B greater than A)
Space (data = 0)
condition
1.5 V to 5 V (A greater than B>
Driver output current capability250 mA

RS485 was able to provide high speed data communications. Although many other formats have taken control, it has been widely used and has been able to meet the needs of a number of applications where data was to be transmitted at high speed for the time being. RS485 has proven to be a solid standard and capable of providing reliable data communications over long distances

Advantages of RS485 and RS422

RS485 Communication and RS422 guidelines differently: two cables are needed for each signal. If you want to transmit four signals, eight cables are required. The following figure shows a single RS485 / RS422 signal being transmitted. To transmit logic 1, line B is high and line A is low. To transmit a logical 0, line B is low and line A is high. The advantage of this arrangement is that the signals can be transmitted faster and at greater distances than is possible with a single cable.

RS485 differential wiring.

Differences between RS422 and RS485

In RS422 and RS485 Communication , one controller can control many receivers. In RS485 Communication, each controller can be turned off allowing multiple units to send data through a single pair of cables. This is not possible in RS422. This additional function causes further problems for RS485 units. Other differences are small, from now on I will refer to units like RS485, but the observations also apply to RS422. Interface units are often described as RS422 / RS485 because an RS485 unit that does not use its additional capacity is RS422 compliant.

RS485 wiring arrangements

When describing that the instruments have an RS485 interface, this does not tell you anything for sure about the signals that are transmitted. Generally, only the transmission (TX) and reception (RX) data from a normal serial port are converted to RS485 or RS422. The other serial port signals are not used. Three arrangements are common: write only, 4 wires (full duplex) and 2 wires (half duplex).

Write-only system: only sending data

In this arrangement, the computer sends data through a single pair of RS485 cables to many instruments. Never reread any data. An example of this is the remote control of CCTV cameras. The operator can send messages to move the camera. Your opinion is the TV picture, so no return information is needed. This only requires the conversion of the RS232 TX to RS485 signal.

Full duplex system: sending and receiving data via 4 cables

Full Duplex means that data can be transmitted simultaneously to and from instruments. This requires 4 cables: one pair to transmit and one pair to receive. The computer alone controls the TX transmission line. Send a message that includes an address. The direct instrument responds only by guiding the RX line.

This means that all connected instruments must use the same software protocol; otherwise there will be confusion about which tool is being addressed. A simple conversion from RS232 to RS485 will suffice for this arrangement and no special requir4 Wire RS485ements are required for general purpose software such as

What is RS232?

RS232C “Recommended 232C standard” is the recent version of the 25-pin standard, while RS232D is 22-pin. In the new 9-pin male D-type PC.

RS232 is a standard protocol used for serial communication The baud rate is used to measure the baud rate. It is described as the number of bits that pass in a second. For example, if the transmission rate is 200, 200 bits are passed per second. On the telephone lines, the transmission speeds will be 14400, 28800 and 33600, it is used to connect the computer and its peripherals to allow the exchange of serial data between them. As you get the strain on the path used for data exchange between devices. It is used in serial communications up to 50 feet with a speed of 1,492 kbps. As defined by EIA, RS232 is used to connect data transmission equipment (DTE) and data communication equipment (DCE). Stop bits are used for a single packet to stop transmission, indicated by “T”. Some typical values ​​are 1, 1.5 and 2 bits.

Parity Bit is the easiest way to check for errors. There are four types, namely odd, marked and spaced. For example, if 011 is a number, the parity bit = 0, or even parity and parity = 1, or odd parity.

RS232 works in bidirectional communication which exchanges data between them. There are two devices connected together, (DTE) Data Transmission Equipment and (DCE) Data Communication Equipment which has pins such as TXD, RXD and RTS & CTS. Now, from the DTE source, the RTS generates the request to send the data. The baud rate is used to measure the baud rate. It is described as the number of bits that pass in a second. For example, if the transmission rate is 200, 200 bits are passed per second. On the telephone lines, the transmission speeds will be 14400, 28800 and 33600 So on the other hand, the DCE, the CTS, paves the way for data reception. After clearing a route, it will report the RTS from the DTE source to send the signal. The bits are then transmitted from DTE to DCE. Now again from the DCE source, the request can be generated by RTS and CTS from DTE sources clarifies how to receive the data and gives a signal to send the data. This is the whole process by which data Stop bits are used for a single packet to stop transmission, indicated by “T”. Some typical values ​​are 1, 1.5 and 2 bits.

RS-485 connections are often used for simple networks and can be connected in 2 or 4 wire mode. In a typical application, multiple devices that support addresses can be connected to a single controlled PC (PC), and therefore a single line can be used for communication. It is also possible to convert between RS485 and RS232 using simple interface converters which can include optical isolation between the two circuits, as well as the suppression of overvoltages for any electrical “spikes” that can be detected.

Using RS-485, it is possible to build a multipoint data communication network. The standard specifies that up to 32 controllers or transmitters can be used together with 32 receivers in a system. This means that there can be 32 nodes capable of transmitting and receiving. This can be further expanded by using “automatic” repeaters and high impedance receivers / controllers. In this way it is possible to have hundreds of nodes in a network. In addition, RS485 extends the common mode range for both tri-state controllers and receivers with the power turned off. In addition, RS-485 controllers can support “data collision” (bus containment) problems and bus failure conditions.

As the RS485 networks get bigger, the problem of data collisions increases. This can be solved, at least in part, by ensuring that the hardware units (converters, repeaters, microprocessor controls) are designed to remain in reception mode until they are ready for data transmission.

Another approach is to design a “single teacher” system. Here the master initiates a communication request to a “slave node” addressed to that unit. The hardware detects the transmission start bit and then enables the transmitter. Once the requested data is sent, the hardware goes back to receiving mode.

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