Internet Explorer is not a supported browser for TI.com. For the best experience, please use a different browser.
Video Player is loading.
Current Time 0:00
Duration 19:28
Loaded: 0.00%
Stream Type LIVE
Remaining Time 19:28
 
1x
  • Chapters
  • descriptions off, selected
  • en (Main), selected

Welcome to an overview presentation discussing RS-485 technology. TIA/EIA-485A or RS-485 is a differential signaling standard which defines the electrical characteristics of drivers and receivers used to implement a balanced multipoint transmission line. The standard is intended to be referenced as a physical layer for higher-level standards such as DLT-645, DMX-512, Modbus, and others and is widely used in industrial applications for its robust electrical characteristics.

RS-485 can connect data terminal equipment in a point-to-point multidrop or multipoint network architecture over long distances, communicating at data rates up to 50 megabits per second. RS-485 is a balanced transmission standard, meaning it requires two signal lines whose voltages are inverse of one another.

This offers two advantages for signal integrity, first because the two signal lines are implemented with twisted pair cable, noise from external sources couples equally into both signal lines as common mode noise, which is rejected by the differential receiver. Second, because the two signal lines switch inversely to one another, the electromagnetic field emitted by each line is opposite of the other and cancels each other.

RS-485 is the ideal technology for communicating in industrial applications for several reasons. First, as discussed previously, the RS-485 receiver has the ability to reject large amounts of common mode noise that may couple onto the signal lines via an external oscillator or noise source. Second, RS-485 allows for bidirectional communication over a single twisted pair cable from DC to 50 megabits per second.

Third, an RS-485 driver is required to drive a large differential signal into an RS-485 load. This enables the signal to travel long distances while still being large enough to be accurately interpreted by the receiver. Finally, an RS-485 driver and receiver are also required to operate over a common mode range of minus 7 volts to plus 12 volts. This enables data fidelity in the presence of large ground potential differences relative to other interface standards and may also extend the operable distance of an RS-485 transmission line.

RS-485 networks can be designed using a half duplex topology where all of the drivers and receivers of a given network share the same twisted pair cable or in a full duplex topology where the drivers and receivers are mixed amongst two sets of twisted pair cable. In the half duplex topology, only one signal pair is used for acquiring the driving and receiving of data to occur at different times. Having more than one driver accessing the bus at the same time leads to bus contention, which at all times must be avoided through software control.

The full duplex implementation requires two signal pairs of four wires and full duplex transceivers with separate bus access lines for transmitter and receiver. The full duplex topology allows a node to simultaneously transmit data on one pair while receiving data on the other pair. Both implementations necessitate the controlled operation of all nodes via direction control signals such as driver receiver enable signals to ensure that only one driver is active on the bus at any time.

The RS-485 standard suggests that nodes be networked in a daisy chain architecture. In this architecture, the participating drivers, receivers, and transceivers connect to a main cable trunk via short network stubs. There are other non-ideal architectures that can be realized with RS-485 such as star, ring, and backbone with start topologies, but these can introduce reflections and standing waves on the bus, which degrade the overall signal integrity of the network.

In order to achieve reliable communication in these non-ideal topologies, the data rate must be drastically decreased. Reducing the data rate increases the bit time and helps to negate the effects of reflections and standing waves as the longer bit time allows the ringing on the bus to subside before the sample point. If a non-ideal network topology must be used, it is best to insert a repeater node at each junction.

The RS-485 differential driver is implemented as an H-bridge output stage that drives current from terminal A to terminal B and vice versa, depending on the logic state of the data input D. To estimate the driver differential output voltage for a given load, assume each high and low side transistor have an on resistance of between 20 and 30 ohms.

The RS-485 differential receiver consists of a resistive divider network on each of the A and B inputs, which serves two functions. First, the resistors R2 and R3 bias the comparator inputs relative to the local supply voltage and ground. This allows the comparator to evaluate the differential voltage between A and B without the need for a connection between the driver ground and receiver ground.

Second, it attenuates signals that are beyond the range of the supply voltage of the receiver to levels that are within the supply range. Because of ground potential differences that may exist between transceivers on the bus, voltages as low as minus 7 volts and as high as plus 12 volts may appear at the A and B terminals of the device. The attenuation factor is typically on the order of 10 to 1, so the voltage levels that actually appear at the comparator input are within the operating range of the device.

The TIA/EIA-485A standard defines that RS-485 compliant drivers be capable of providing a minimum differential output voltage of 1.5 volts across a 54 on load. The standard also defines that the RS-485 receiver input threshold sensitivity be plus and minus 200 millivolts. These two values provide sufficient margin for a reliable data transmission even under severe signal degradation across long cable lengths and connectors. This robustness is the main reason why RS-485 is well suited for long distance networks in noisy environments.

Data transmission lines should always be terminated, and stubs should be as short as possible to avoid signal reflections and standing waves on the signal lines. Figure 1 shows and an unterminated network which is not recommended. In order to achieve reliable communication in unterminated networks, the data rate must be drastically decreased. As stated earlier, reducing the data rate increases the bit time and helps to negate the effects of reflections and standing waves as the longer bit time allows the ringing suppression on the bus to subside before the sample point.

Figure 2 shows proper termination, which reduces the matching of the termination resistors, RT, to the characteristic impedance Z naught of the transmission cable. Because RS-485 recommends cables with characteristic impedance of 120 ohms, the cable trunk is commonly terminated with 120 ohm resistors, one at each end. This reduces reflections and standing waves on the bus by absorbing the energy on the bus during normal operation.

Figure 3 shows another option for terminating an RS-485 network, which is ideal for applications in noisy environments. In this scheme, the 120 ohm resistors are replaced by two 60 ohm low-pass filters to provide additional common mode noise filtering. It is important to match the resistance values, preferably with 1% precision resistors to ensure equal roll off frequencies of both filters. Large resistor tolerances, like 20%, cause the filter corners to differ and the common mode noise to be converted into differential noise, thus compromising the receiver's noise immunity.

The distance between the main RS-485 bus and the node is the stub. As a rule of thumb, the electrical length of a stub should be shorter than 1/10 of the RS-485 driver's output rise time or the stub length can be calculated through the provided equation. The given table shows the maximum stub length for various Texas Instruments RS-485 transceivers based on their empirical rise time data and a cable with the velocity factor of 78%.

Based on this data, it can be seen that drivers with a long rise time are well suited for applications requiring long stub lengths. The driver rise time also directly correlates to EMI performance so applications that are sensitive to noise should look to implement transceivers with a slower slew rate.

Because a stub length begins where the node connection branches off the main cable trunk, the stub links between a daisy chain network and a backbone network using junction boxes can differ significantly. Fail-safe operation is a receiver's ability to assume a determined output state in the absence of an input signal. Three possible causes can lead to the loss of signal. One, open circuit caused by a wire break or the disconnection of a transceiver from the bus. Two, short circuit caused by an insulation fault connecting the wires of one bus to another.

Three, idle bus occurring when none of the bus drivers are active. Because these conditions can cause conventional receivers to assume random output states when the input signal is 0, modern transceiver designs include biasing circuits for open circuit, short circuit, and idle bus fail-safe that force the receiver's output to a determined state under a loss of signal condition.

A drawback of these fail-safe designs is their worst case noise margin of less than 100 millivolts, thus requiring external fail-safe circuitry to increase noise margin for applications in very noisy environments. An external fail-safe circuit consists of a resistive voltage divider that generates sufficient differential bus voltage to drive the receiver output into a determined state. To ensure sufficient noise margin, VAB must include the maximum differential noise measured in addition to the 200 millivolt receiver input threshold.

Because a driver's output depends on the current and must supply into a load, adding transceivers and fail-safe circuits to the bus increases the total load current required. To estimate the maximum number of bus loads possible, RS-485 specifies a hypothetical term known as the unit load, which represents the load impedance of approximately 12k ohms.

Standard compliant drivers must be able to drive 32 of these unit loads. Today's transceivers, however, often provide a reduced unit load such as 1/8 unit loading, thus allowing the connection of up to 256 transceivers on the bus. Because fail-safe biasing contributes up to 20 unit loads of bus loading, the maximum number is decreased.

RS-485 applications benefit from differential signaling over twisted pair cable, because noise from external source couples equally into both signal lines as common node noise, which is rejected by the differential receiver input. Industrial RS-485 cables are of the sheathed unshielded twisted pair type with the characteristic impedance of 120 ohms and typically of 22 to 24 American wire gauge.

The maximum bus length is limited by the transmission line loss and the signal jitter at a given data rate. The provided plot shows cable length versus data rate characteristics of a conventional RS-485 cable for a 10% signal jitter. Section 1 of the graph presents the area of high data rate over short cable length. Here the losses of the transmission lines can be neglected, and the data rate is mainly determined by the driver's rise time.

While the standard recommends 10 megabits per second, today's fast interface circuits can operate at data rates of up to 50 megabits per second. Section 2 shows the transition from short to long data lines. Now the losses of the transmission line have to be taken into account, thus with increase in cable length, the data rate must be reduced. A rule of thumb states that the product of the line length times the data rate should be smaller than 3 times 10 to the seventh.

Section 3 presents the lower frequency range or the line resistance and not the switching limits of the cable length. Here the cable resistance approaches the value of the termination resistor. This voltage divider diminishes the signal by 6 dB.

When designing a remote data link, the designer must assume that large ground potential differences exist. These voltages add as common mode noise to the transmitter output. Even if the total superimposed signal is within the receiver's input common mode range, relying on the local earth ground as a reliable path for the return current is dangerous.

Because remote nodes are likely to draw their power from different sections of the electrical installation, modifications to the installation during routine maintenance work can increase the ground potential difference to the extent that the receiver's input common mode range is exceeded, thus a data link working today might cease operations some time in the future. When possible and designed, use a ground wire.

The direct connection of remote grounds through a ground wire is also not recommended, as this causes large ground loop currents to couple into the data lines as common mode noise. To allow for a direct connection of remote grounds, the RS-485 standard recommends the separation of device ground and local ground via the insertion of resistors. While this approach reduces loop current, the existence of a large ground loop keeps the data link sensitive to noise generated somewhere else along the loop, thus a robust data link has not yet been established.

Here are some RS-485 tools and reference designs to help you get started designing or taking your designs to the next level. There are three different evaluation modules for evaluating Texas Instruments RS-485 transceivers. The first is the half duplex EVM, which supports the eight-pin SOIC transceivers.

Next, there is the SN65HVD22EVM, which supports both the eight-pin SOIC half duplex transceivers and the 14-pin SOIC full duplex transceivers. And finally, there is the full duplex EVM, which supports only the 14-pin SOIC transceivers. All three of these EVMs can be acquired through the Texas Instruments website under Tools and Software on the RS-485 Interface Products page.

Many industrial communication networks require the distribution of both power and data lines between master and remote slave nodes. As the distance between nodes increases, the cost of cabling to increases as well. Combining power and data communication onto a common pair of wires reduces the overall number of wires connecting each node and can provide significant cost savings. It also can help with the installation process of new systems into applications with an existing cabling infrastructure that were designed for data only or power only distribution.

The TIDA-00527 reference design uses a bias T structure in order to combine power and data. High-frequency data, in this case from an RS-485 transceiver, is connected to the distribution line through a series capacitor, which allows data to pass through while protecting the transceiver from large DC potentials on the bus.

A power supply is connected to the line through an inductor, which allows for DC power to be distributed through the line and filtered out at the other end. Note that this technique requires that the data signals being transmitted do not have content at DC or at very low frequencies. The series capacitor will form a single pole high-pass filter, which with the shunt termination resistance at each end of the bus, and the lowest signal frequency should be higher than the filter cutoff. Manchester encoding is a common way to eliminate the DC portions of the data signal, although other encoding schemes like 8b/10b could be used as well.

The TIDA-01171 reference design details the AC coupling of RS-485 signals. Although these signals are typically DC coupled, using AC coupling has several advantages. AC coupling allows transceivers to operate with large common mode offsets, enabling long-distance communication without the routing of an additional ground wire. AC coupling also protects the transceiver against high-voltage DC defaults. For example, if the differential bus is shorted to the power supply. This protection is useful in industrial applications that require data transfers across long distances with a minimum number of wires.

The TIDA-01090 reference design allows for the direction of communication on an RS-485 bus to be set automatically when a node needs to transmit data. The transceiver enables the driver for a configurable length of time when data is sent to a transceiver from a host microcontroller or universal asynchronous receiver transmitter or UART. This eliminates the need for manual control of the driver enable of the transceiver and the need for the receiver enable controls, reducing the number of logic pins needed to implement the RS-485 interface.

The TIDA-01365 reference design implements a one to four or four to one bi-directional RS-485 fanout hub with a DC to DC converter for taking a 24-volt industrial supply to the required 5 volts for the RS-485 transceivers and microprocessor. The design also features an automatic direction control circuit, which enables the driver for a configurable length of time when data is sent to a transceiver device from a host MCU or UART.

This eliminates the need for manual control of the driver enable of the transceiver and the need for the receiver enable controls, reducing the number of logic pins needed to implement the RS-485 interface. The design techniques detailed in TIDA-01365 can be implemented in the more difficult star ring and backbone with star network topologies to guarantee data fidelity.

Thank you for your time today. And please let the Texas Instruments Industrial Interface Applications Team and Business Development Team assist you with your questions and your next design by emailing us at these mailing lists.