Are you looking for more information about RS-485? Based on feedback from the TIE2E™ community, we've compiled a list of the most frequently asked questions regarding design challenges for isolated RS-485 transceivers. We hope this list will provide you with useful insights into RS-485 isolated signals and power supplies.
1. When is it necessary to isolate the RS-485 bus?
Isolation prevents direct current (DC) and abnormal alternating current (AC) between two parts of a system, while still allowing signal and power transmission between them. Isolation typically protects electrical components or personnel from dangerous voltage and current surges; isolation used to protect personnel is called enhanced isolation. Isolation prevents ground loops from forming during long-distance communication between nodes. Isolation also allows for a much higher rate of change in inter-node communication ground potential difference than recommended by the RS-485 standard.
2. How many nodes can be connected to a single RS-485 bus?
To estimate the maximum possible bus load, RS-485 defines a hypothetical term "unit load (UL)," which represents a load impedance of approximately 12kΩ. The Electronic Industries Association (TIA/EIA) RS-485 standard mandates a maximum of 32UL loads that can be added to a single RS-485 bus. We calculate the UL of a node using the worst-case performance ratio obtained by dividing the input voltage by the leakage current, as shown in Equation 1.
Once the UL of the node is calculated, the maximum number of nodes can be calculated using Equation 2:
Most TI isolated RS-485 transceivers have a single UL of 1/8, so the conversion result is that there can be a maximum of 256 nodes on an RS-485 bus.
For more information on UL and the number of nodes that can be connected to a single RS-485 bus, please refer to the article "RS-485 Unit Load and Maximum Number of Bus Connections" in the Analog Applications Journal.
3. What is the speed-length correlation of isolated RS-485?
There is an inverse relationship between signal rate (speed) and cable length. The exact relationship depends on the resistance and capacitance of the cable itself. When building an RS-485 network, cable selection is as important as the transceiver to ensure reliable communication over the required distance. Figure 1 illustrates the correlation between signal rate and cable length. Circle 3 in the figure shows the maximum cable length independent of signal rate; in this part of the curve, the DC resistance of the cable causes signal attenuation, limiting the maximum communication distance. Circle 2 shows the inverse relationship between signal rate and cable length, caused by transmission line losses, which increase with longer cables. Circle 1 allows you to ignore transmission line losses; the rise and fall times of the driver determine the main limiting factors for the maximum data transfer rate. The RS-485 standard recommends a maximum signal rate of 10 Mbps; however, with today's technology, signal rates have reached 50 Mbps.
Figure 1: Relationship between signal rate and cable length
4. What is fault protection bias, and how is it designed?
To comply with the RS-485 standard, the receiver output must generate a logic high level when the differential input (VID) exceeds 200mV, and a logic low level must be output when VID is below -200mV. However, an invalid output will be generated in the following three cases:
Bus open circuit, for example, cable breakage or connector disconnection.
A short circuit in the bus, such as a break in the cable insulation, causes the stranded wires to short-circuit.
Bus idleness occurs when there is no active driver on the bus.
In any of the above situations, for terminated transmission lines, the VID of the RS-485 receiver is zero, while the output of a fault-free receiver is uncertain.
Fault protection bias provides a differential voltage to the idle bus, thus maintaining the receiver in a logic high state. Without fault protection bias, terminating resistors can drop the bus voltage to 0V, leading to erroneous outputs or signal oscillations. You can design fault protection bias by combining a resistor network with the RS-485 receiver. TI's isolated RS-485 transceivers all integrate fault protection bias for open-circuit, short-circuit, or idle bus conditions, eliminating the need for external circuitry to design fault protection bias.
5. When is it necessary to terminate the RS-485 bus, and what are its advantages and disadvantages for the system?
In most RS-485 applications, terminating resistors are matched to the characteristic impedance of the cable to prevent signal reflection. Terminating resistors should be installed at both ends of the RS-485 bus. Some short-distance communications can operate without terminating resistors, but terminating the line is the best method for all applications. The disadvantage of terminating resistors is the DC loss they introduce, which can lead to higher power consumption in the system. Even with this drawback, terminating resistors remain the best choice for the vast majority of applications.
6. What type of transient protection is required for isolated RS-485 devices?
The type of transient protection employed by isolated RS-485 devices depends on the type of interference likely in the final system, such as electrostatic discharge (ESD), electrical fast transient/burst (EFT), or surge, and the required level of protection. TI's isolated RS-485 transceiver product family, if featuring a floating insulated ground conductor, provides some level of internal transient protection at the integrated transceiver bus termination. Furthermore, with proper system design, you can utilize the isolation barrier to create a high impedance against these transient changes. If you do not want differential transients in your system and have tested all transients related to the final device ground, connecting the protective ground (PE) to the logic side of the isolated transceiver will cause all high-voltage transients to occur at the isolation barrier. Connecting PE to the logic side eliminates the need for external components such as transient voltage suppression (TVS) diodes or pulse-damping resistors.
Figure 2 illustrates the enhanced transient protection technology using ISO 1410.
Figure 2: Half-duplex isolated RS-485 transceiver with optional bus protection components.
7. How do you construct an isolated power supply for an isolated RS-485 node?
There are several options for constructing an isolated power supply for an isolated RS-485 node; the best solution depends on the specific application requirements.
One option is to use a transformer driver like the TISN6501, which can be used in push-pull configurations with a secondary-side transformer and a rectified low-dropout regulator (LDO). The SN6501 offers up to 1.5W of power and can be used as an isolated power supply. This device is highly flexible and can be used in virtually all applications. This is because the transformer turns ratio allows the power supply to provide the necessary isolation and output voltages. If you need to provide isolated power to other devices, you can use the SN6505 instead of the SN6501, achieving up to 5W of output power. The SN6505 features additional protection characteristics such as overload and short-circuit protection, thermal shutdown, soft start, and slew rate control, allowing designers to construct robust solutions.
Figure 3: Constructing an ISO1410 isolated power supply using SN6501
Another option for space-constrained applications is the ISOW78xx series of devices, which provide signal and power isolation in a small SOIC-16 package. The ISOW7841 can be used in conjunction with non-isolated RS-485 transceivers, as shown in isolated RS-485 transceivers with integrated signal and power reference designs. This combination offers a small footprint, eliminates the need for transformers, and facilitates certification. Please refer to all documentation in the ISOW7841 online product folder for more details.
Figure 4: Using ISOW7841 to build an isolated RS-485 with integrated signal and power.
