Inside an isolated RS-485 transceiver

-May 23, 2017

Isolation is a means of preventing current flow between two communicating points, while allowing data and power signal transmission between them. Isolation prevents high voltages from damaging sensitive electronic components or harming humans. It also eliminates ground loops in communication links with large ground potential differences to maintain signal integrity.

Over the past decade, legislation has changed and now requires machinery and equipment operating in harsh environments to implement isolation for their data transmission systems. As a result, the trend away from legacy single-channel isolated systems to applications utilizing multi-channel isolation has led to the introduction of new isolation components. Many of these applications involve data communication in telecommunication and industrial networks, medical systems, sensor interfaces, motor control and drive systems, and instrumentation.

This article focuses on isolated digital interfaces conforming to RS-485, which continues to be the industry’s workhorse standard for data transmission. We’ll provide an overview of RS-485’s common-mode voltage range (CMVR) definition, and explain how isolating a transceiver’s signal and supply paths from the local controller circuit allow it to tolerate huge common-mode voltages. Finally, we will present a new RS-485 isolator based on giant magnetoresistance (GMR) technology, and discuss its benefits over other isolation technologies.

Common-mode voltage range

The RS-485 standard specifies its common-mode voltage range from −7V to +12V. Figure 1 shows this range, including the driver output common-mode voltage (VOC), the ground potential difference between driver and receiver grounds (GPD), and the longitudinally coupled noise (VN).

Figure 1 VCM in a non-isolated RS-485 data link: VCM = VOC + GPD + VN

The driver is designed to generate a symmetrical, differential output (VD) around a common-mode component of VCM = VCC/2, thus making the line voltage at one output VA = VCC/2 ± VD/2, and the voltage at the complementary output VB = VCC/2 VD/2.

The receiver is designed to process only differential signals within the specified CMVR and to reject any common-mode components. This is accomplished by an internal voltage divider that equally attenuates both common-mode and differential signals (Figure 2). The subsequent differential comparator then builds the difference between the two attenuated input signals, thus amplifying only the differential component.

Since the voltage dividers represent common-mode resistances (RCM) between each receiver input and receiver ground, the entire common-mode voltage of the data link drops across these resistances. This means that for a standard transceiver, the receiver must correctly detect differential input voltages over the entire CMVR from −7V to +12V.

To tolerate higher common-mode voltages (VCM), such as ±25V, the transceiver bus I/O stages are redesigned such that the driver output transistors possess higher stand-off voltages and the receiver voltage dividers have higher divider ratios, which can also necessitate higher resistor values.

For very high common-mode voltages (in the hundreds of volts), the insertion of galvanic isolation barriers is required, which eliminate high voltages at the transceiver bus terminals.

Figure 2 Receiver-equivalent circuit diagram (a), its common-mode representations (b), and a further simplified VCM equivalence (c)


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