Whether the RS-485 transceiver conundrum has you tossing and turning, we all tell you!

Questions about RS-485 transceivers have been bothering you for a long time? do not worry! ? This article provides some insights based on frequently asked questions within the Texas Instruments online support community E2E™, and for those of you who want to understand this established communications standard!

1. When is termination required on an RS-485 bus, and how do I properly terminate it?

RS-485 bus termination is useful in many applications because it helps to improve signal integrity and reduce communication problems. “Termination” refers to matching the characteristic impedance of the cable to the termination network so that the receiver at the end of the bus can observe the maximum signal power. An unterminated or improperly terminated bus will not be well matched, resulting in reflections at the end of the network, resulting in reduced overall signal integrity.

When the bidirectional loop time of the network is much greater than the signal bit time, termination is not necessary because each time reflections reach the end of the network, they lose energy. However, for applications where the bit time is not substantially longer than the cable loop time, termination is critical to minimize reflections.

The most basic termination is called a parallel termination and consists of a single resistor, as shown in Figure 1. The RS-485 standard requires a nominal characteristic impedance of 120Ω, so the default value for the termination resistor should be RT = 120Ω. Read the blog post: “RS-485 Basics: When You Need Termination, and How to Do It Right. “

Figure 1: RS-485 bus with parallel termination

2. What is fail-safe biasing and how is it implemented?

Fail-safe biasing is a way to ensure that the RS-485 receiver is not left in an indeterminate state due to differential input voltages. The Electronic Industry Alliance (EIA)-485 standard points out: when the differential voltage is ≥ + 200mV, the input threshold of RS-485 is logic high; when the differential voltage is ≤ -200mV, the input threshold of RS-485 is logic low, so that the high and low thresholds 400mV indeterminate state is maintained between.

Fail-safe biasing can be achieved in two ways:

  • Choose a transceiver with built-in fail-safe input thresholds in the receiver.

  • Add external resistors to create external bias on the unloaded bus.

Both methods ensure a logic high state on the bus. Read the blog post: “RS-485 Basics: Two Approaches to Fail-Safe Biasing Networks.

3. How to calculate the maximum number of nodes on the RS-485 bus?

RS-485 is a multipoint differential bus, which means that all nodes on the bus share a common transmission medium. As the total number of nodes increases, so does the load on each drive.

The Telecommunications Industry Association (TIA)/EIA-485 standard created an assumed unit load (UL) to help calculate the maximum number of nodes on an RS-485 bus. The standard stipulates that the driver must be able to drive at least 1.5V differential signals in parallel on up to 32 unit loads, with two 120Ω termination resistors connected to each end of the bus.

Equation 1 represents the worst-case ratio of the input voltage divided by the leakage current to calculate the input resistance. After the input resistance of the nodes is determined, the maximum number of nodes on the RS-485 bus can be calculated using Equation 2:

Input Resistance = Maximum (VIN/Ileakage) (1)

Number of nodes = 32 / input resistance (2)

4. How do I need to add a ground wire between nodes at the right time?

When designing a remote data link, some ground potential difference must be assumed. These voltages add common-mode noise, Vn, to the transmitter output. Even if the total superimposed signal is within the input common-mode range of the receiver, it is not safe to rely on the local ground potential difference as a reliable path for the return current. Proper grounding techniques are required when the ground potential difference (GPD) exceeds the common mode range of the receiver (which often occurs with long cables and high current loads).

Figure 2: Remote Node Configuration: Separate Ground Points (a); Directly Connected Remote Grounds (b);

Separation of device ground and local system ground (c)

Figure 2a shows a remote node that may draw power from different parts of the electrical installation. Any changes to the installation, such as during maintenance work, can push the GPD beyond the receiver’s input common mode range. As a result, a currently working data link may stop functioning in the future.

It is also not recommended to connect the remote ground directly through the ground wire (Figure 2b), as a direct connection can cause large ground loop currents to couple into the data lines as common-mode noise.

To enable direct connection of remote grounds, the RS-485 standard recommends separating the equipment ground from the local system ground by inserting a resistor (Figure 2c). While this approach reduces loop current, the presence of a large ground loop keeps the data link sensitive to noise generated elsewhere in the loop. Therefore, a stable data link has not been established.

To withstand GPDs up to several thousand volts over long distances on a robust RS-485 data link, the best approach is to galvanically isolate the bus transceiver’s signal and power lines from its local signals and power. In this case, power isolators (such as isolated DC/DC converters) and signal isolators (such as digital capacitive isolators) prevent current flow between remote system grounds and avoid current loops.

5. What is the recommended length and speed for RS-485?

At the rated data rate, the maximum bus length is limited by transmission line losses and signal jitter. Figure 3 shows the cable length versus data rate characteristics of a conventional RS-485 cable at 10% signal jitter due to the sharp drop in data reliability at jitter at 10% or more baud rate.

Figure 3: Cable Length vs. Data Rate Recommendations

On Figure 3, the circle marked with number 1 represents the high data rate area when the cable length is short. Here, the loss of the transmission line can be ignored. The data rate is primarily determined by the drive’s rise time. Although the standard recommends 10 Mbps, today’s fast interface circuits can operate at data rates as high as 50 Mbps.

The red number 2 in Figure 3 represents the transition from a short data line to a long data line. Losses in longer transmission lines must be considered. Therefore, as the cable length increases, the data rate must decrease.As a rule of thumb, the line length[m]with data rate[bps]The product of should be 107.

The red 3 represents the low frequency range where the interaction of cable series resistance and line termination can cause signal attenuation. At some point, the amplitude of the signal becomes smaller than the receiver can normally detect (ie, does not exceed the VIT threshold).

6. How to estimate the power consumption of RS-485?

To calculate power consumption, the power can be divided into several parts. When the device is powered up with no external load, the power dissipation goes to the integrated circuit itself. If a load is added to the output pin, the power to drive the load is drawn from the device. Since RS-485 has differential signaling, the load is usually added between the A and B pins.

In Figure 4, the blue trace, PDic, is the power dissipated by the device. For low data rates, the power dissipation is mainly from the resistive load (red trace), PDdc. For high data rates, the power dissipation (green trace) PDac of the capacitive load needs to be considered.

Figure 4: Calculating Power Sectors

Equation 3 calculates the total power dissipation as:

PDtotal = PDic + PDdc + PDac (3)

To calculate the total power consumption, the power of each part must first be calculated. Device power dissipation is shown in Equation 4, where the quiescent supply current, Icc, is specified in the data sheet:

PDic = Vcc*Icc(4)

If a resistive load is placed on the bus, the driver develops a voltage (Vod) across it, as shown in Equations 5 and 6, where C is the parasitic capacitance, which includes the transceiver’s capacitance, the load’s capacitance, and the trace capacitance . The data frequency f is also included in the calculation.

PDdc = Vcc*I – I2*R = (Vcc – I*R)*I (5)

PDac = 2*2C*f*Vcc*Vod(6)

Read the blog post: “How to Calculate Power Consumption in High Speed ​​RS-485 Transceivers”.

7. How to protect the RS-485 interface from electrostatic discharge (ESD)?

There are many types of ESD protection, including Human Body Model, International Electrotechnical Commission (IEC) contact discharge, and IEC air-gap discharge. If a transceiver has integrated IEC ESD (such as Texas Instruments’ THVD1450 or THVD1500), the RS-485 interface can be protected from ESD at the level of the specified transceiver without the need for external components.

For example, the THVD1450 can support 18-kV IEC 61000-4-2 contact discharge without any external components. Many devices on the market do not have this integration and therefore require external transient voltage suppression (TVS) diodes. Read the blog post: “How to Select TVS Diodes for RS-232, RS-485, and CAN Based on Voltage Rating.”

8. How do I know if an external TVS diode is required?

Industrial networks must operate reliably in harsh environments. Electrical overstress transients caused by ESD, switching of inductive loads, or lightning strikes can disrupt data transmission and damage bus transceivers unless effective measures are taken to reduce the effects of the transients.

Texas Instruments devices have been tested to the following standards:

  • l IEC 61000-4-2 ESD antistatic test, which simulates the operator applying static electricity directly to adjacent electronic components. THVD1500 and THVD1450 have passed this standard test.

  • l IEC 61000-4-4-4 Electrical Fast Transient (EFT) or Burst Immunity Test, simulating everyday switching transients caused by inductive load interruptions, relay contacts bouncing, etc. THVD1450 and THVD1550 have passed this standard test.

  • l The surge immunity test IEC 61000-4-5 is the most stringent transient immunity test in terms of current and duration, approximately 1000 times longer than ESD and EFT tests. THVD1429 and THVD1419 have passed this standard test.

The latest RS-485 transceivers from Texas Instruments’ THVD series integrate various protection levels according to these standards and do not require additional external protection. Protection levels are specified in the device data sheet.

9. How to prevent high voltage short circuit?

In many RS-485 applications, there is a risk of inadvertently connecting communication lines to power lines. This risk is especially high in field-installed systems such as HVAC systems, lighting controls, or other building automation applications. In these cases, it is imperative to ensure that the RS-485 transceiver is not damaged to avoid the risk of costly field returns and reinstallation.

While clamping elements such as TVS diodes can limit the maximum voltage observed by the transceiver during transient events, they are often unable to protect against prolonged stress (such as a DC short). To prevent these conditions, some series current limiting components are required. A typical approach is to use positive temperature coefficient (PTC) resistors. This resistor has a low resistance under nominal conditions, but under fault conditions, when a large current flows (for example, through a clamping device such as a TVS), the resistance becomes large. Texas Instruments Reference Design”Reference Design for Protecting RS-485 Transceivers from Sustained High Voltage/Electrical OverstressAn example of an implementation using the THVD1500 transceiver can be seen in .

However, using these additional series current limiting and parallel voltage clamping components can be expensive and take up valuable PCB space. Therefore, in most cases, the more optimal approach is to use transceivers that can withstand these high fault voltages without external protection. The THVD2450 is an example. It is rated to withstand DC short circuit voltages up to +/- 70V.

For specific information on isolated RS-485 transceivers, check out the FAQ blog post: “Seven Questions About Isolated RS-485 Transceivers.”

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