Understanding critical bus voltages for RS-485 transceivers

Due to its robustness and high reliability, RS-485 has become the most commonly applied interface technology in noisy industrial environments worldwide. The trend towards wider operating ranges combined with higher stand-off capabilities has produced modern transceiver designs whose performance exceeds the initial RS-485 standard (EIA/TIA-485) by far.

New transceiver specifications outlining these performance improvements in component data sheets, however, are often misunderstood by the end user – the system designer. For example, typical confusion exist when comparing a transceiver’s maximum voltage levels given in the Absolute Maximum Ratings (AMR) section with those provided under Recommended Operating Conditions (ROC).

Questions often arise such as: Will the transceiver transmit data reliably under absolute maximum rating conditions? Why is the input voltage under ROC much smaller than the one in AMR? How is the common-mode voltage range defined?

Because transceiver data sheets rarely provide detailed explanations of these parameters, this article tries to rectify this shortcoming. First we explain the basic operation of a transceiver, then how to derive from there the common-mode voltage term. Finally we distinguish between recommended and maximum operating conditions.

Basic transceiver operation

The Driver

A transceiver’s driver section consists of an H-bridge output stage. A logic high at data input, D, turns on transistors Q2 and Q3. This drives a current from the A to the B terminal. Logic low at D turns on Q1 and Q4 and drives a current in the opposite direction, from B to A.

The internal resistances of the reverse leakage protection diodes and in combination with the transistor on-resistances form a voltage divider with the external differential load resistance, RD. This causes the line voltages VA and VB to be significantly smaller than the supply voltage, VCC (Figure 1).

The resulting, complementary line voltages, VA and VB, represent two dc-voltage levels that swing around an average dc-level that is common to both bus lines. This voltage is known as the driver common-mode output, or output offset voltage, VOS.

This common-mode voltage can be expressed through, while the differential output voltage is. Equating both equations and solving for the line voltages yields and. Thus, the driver can be represented as a signal source with a common-mode component, VOS that is superimposed by the complementary differential components.

The Receiver

A transceiver’s receiver section contains a resistive voltage divider, R1 / R2, at each input, followed by a comparator. The voltage dividers attenuate the input signals, VA and VB, by a ratio of 10:1. This ratio determines the maximum applicable receiver input voltages at A and B.

Because the voltage dividers are referenced to receiver ground, both, the common-mode and the differential voltage component are attenuated by the same ratio. The differential comparator then removes the common-mode signal and only reacts to the differential input signal, VID = VA – VB. Thus, in standard RS-485 transceivers output, R, turns high when VID 200 mV, and low when VID < –200mV.

Figure 2 shows that each receiver input has a common-mode input resistance of RCM = R1 + R2, and a differential input resistance of RIN = 2 RCM. RCM causes common-mode loading for each bus line, that is, the common-mode voltage on both lines drives current through RCM towards receiver ground. In contrast RIN causes differential load currents to flow between the bus lines.

Maximum receiver input voltage range

The RS-485 standard specifies a common-mode input range from –7V to +12V and a maximum ground potential difference (GPD) between a driver and a remote receiver of ±7V. Figure 3 shows a data link example for maximum possible bus voltages. In this case the link is an unterminated point-to-point connection. The single receiver presents light loading, allowing the line voltages to swing across the entire supply rail.

One extreme case is when the driver outputs 5V with respect to driver ground (GNDD) while the GPD = –7V. In this case the maximum positive receiver input voltage is +12V with respect to receiver ground (GNDR). The other extreme is when the driver outputs 0V with respect to GNDD while the GPD = +7V. In this case the maximum negative receiver input is –7V with respect GNDR.

To ensure reliable data transfer the maximum receiver input voltage range spans from +12V to –7V. Note, the GPD is the difference between driver and receiver ground potential and represents a second common-mode voltage that adds to the driver output offset. The total common-mode voltage, VCM, for a data link is given with VCM = VOS + GPD.

A further requirement of RS-485 is that a driver must be able to drive 32 unit loads over the full GPD of ±7V. A unit load represents a common-mode load and is defined as the dc input current of 1mA at a dc input voltage of 12V. This yields a dc input resistance of 12kohm. Legacy transceivers typically have 1UL common-mode loading, while modern components often possess 1/8 UL, that is one-eighth the legacy input current, or an eight times higher input resistance to ground. To test for maximum common-mode loading, the test circuit in Figure 4 is commonly used.

Here the 60ohm differential resistance represents the two 120 termination resistors at each bus end. The 375ohm resistors resemble the resistance of 32 unit loads, which could be 32 x 1UL transceivers or 256 x 1/8 UL transceivers. The symmetrical GPD of ±7V is replaced by a test voltage ranging from –7V to +12V to test for symmetrical receiver input performance. For example, if the driver is capable of providing the full supply range at its outputs, with a 5V output and VTEST = –7V, the receiver input voltage would be +12V with respect to receiver ground. For a 0V output and VTEST = +12V, the receiver input would be –12V measured against receiver ground.

The minimum and maximum transceiver input voltages are specified with –12V and +12V, respectively. Because within this voltage range reliable data transmission must be assured, these values are specified under Recommended Operating Conditions (ROC).

Note that these recommended maxima assume a maximum driver output range of 0V to 5V. The actual common-mode voltage range is much smaller. For a 5V transceiver, VCM+ calculates to VCM = 2.5V +7V = +9.5V and VCM– = 2.5V – 12V = –9.5V.

Absolute maximum ratings

In applications where RS-485 data lines run parallel to 24Vdc power lines, safety features must be implemented into the transceiver design to prevent the device from damage if a short circuit fault occurs between the power and data lines due to broken cable insulation, for example.

A common solution is the utilization of high-voltage transistors in the driver output stage that provides sufficient standoff against short high-voltage transients as well as long lasting overvoltage potentials. A common requirement in industrial systems is that a transceiver must withstand a permanent short to a 24V supply cable.

External transient voltage suppressors (TVS) that could prevent the transceiver from 24V damage are not accepted. Instead, any TVS components used must protect the 24V supply first. Unfortunately, the 24V is a nominal value and many supply systems can show variations of up to 35V. This increases the necessary TVS breakdown voltage to some 36V. TVS diodes with such high breakdown typically provide clamping voltages of up to 60V. In order to withstand these high transients voltages, transceivers must provide standoff voltage of more than 65V to 70V.

These voltage s
tandoff voltage levels are specified under Absolute Maximum Ratings. Any higher voltage levels will trigger the transceiver’s internal electrostatic discharge (ESD) protection circuits. However, these ESD circuits are designed for very short transient over-voltages only and cannot protect the devices against long-term electrical overstress.

It is of the upmost importance never to exceed the voltage levels specified in the Absolute Maximum Ratings.

Conclusion

Adhering to the voltage levels given in the Recommended Operating Conditions section assures reliable data transmission across the specified transceiver supply and temperature values.

Voltage levels specified in the Absolute Maximum Ratings section present the maximum values beyond which the device suffers damage. Reliable data transmission seizes long before reaching the absolute maximum ratings.

References

1. Fault-Protected RS-485 Transceivers With Extended Common-Mode Range, Datasheet (SLLS872H), Texas Instruments