The RS-485 communication standard of the Telecommunications Industry Association (TIA) and Electronic Industries Alliance (EIA) was established two decades ago and has been used in a wide variety of applications. RS-485 is the ideal standard for reliable communication between two locations up to 1000 meters away because it uses twisted pairs to send differential signals.
For motor control, factory automation, grid infrastructure and other systems where high voltage may be present, communication between the high voltage and low voltage domains requires isolation of the nodes of the RS-485 system. Isolating the RS-485 node protects the circuit and, in some cases, the operator from high voltages and any harmful voltage transients. The isolation used to protect operators from high voltages is called reinforced isolation and is equivalent to connecting two functional isolation barriers in series. Electrical isolation is also used in the RS-485 system to prevent
There are many ways to isolate signals and power to achieve these system-level benefits. This article describes the different solutions that can be used to isolate RS-485 nodes and weighs the pros and cons between them.
Signal isolation
For RS-485 systems, there are two common methods of isolating signals. The first approach is a discrete solution of digital isolators and RS-485 transceivers. In this solution, the enable (RE, DE), transmit (D) and receive (R) signals are isolated by digital isolators (such as ISO7741) between the MCU and the RS-485 transceiver. Figure 1 shows an example of such a solution using the ISO7741 and an RS-485 transceiver such as the THVD1410. A key advantage of a discrete solution is the flexibility to choose the ideal transceiver for a specific application. However, due to the multi-chip approach, the disadvantage is that it requires additional layout space.