1 Introduction

The RS-485 interface works using differential signaling which is effective in rejecting common-mode noise making it a robust choice for long-distance communication. To make the system work properly, the transmitter and receiver have to be connected correctly - pin A to pin A and pin B to pin B. In some RS-485 applications, however, the bus polarity might not be known. For example, with long cable length the two ports might be wrongly detected in installation. TI’s polarity free RS-485 transceivers like THVD1505 and SN65HVD888 are designed to work in these situations. These two devices can detect the bus polarity automatically, resulting in a RS-485 system independent from the bus wire connection. This feature makes the system more reliable and the installation or debug more efficient. THVD1505 meets the requirements of part 11 Serial Communication Protocol RS-485 standard in the Q/GDW 11179.11-2015 standard, published by the State Grid Corporation of China (SGCC). The certificate can be found on this website.

Talking about polarity-free RS-485 transceivers, it is worth mentioning that THVD8000 and THVD8010 also have the polarity-free feature and have been widely used in power line communications. These two devices are able to modulate RS-485 signals through On-off keying (OOK) modulation to transmit RS-485 signals over either an AC or DC power line. This feature can reduce the number of wires from four to two to greatly reduce the system cost. More information about the parts can be found in this article.

Back to the topic of this article, THVD1505 and SN65HVD888 are easy to use in systems. The A and B pins can be connected to the bus freely. However, for the devices to work in a system correctly, some conditions must be met. In a simplified summary, the devices need to be put in receiver mode to detect the existing polarity of the bus. The bus bias can be established by a common non-polarity-free RS-485 transceiver like THVD1550 or external biasing network using pull-up and pull-down resistors. Please be aware that the devices constantly drive the bus with power on and driver enabled.

To help better understand the device’s behavior in applications, several different test cases are presented.