By Bryson Barney
I visited a company who supplied HVAC system modules. In some buildings their system operated well with no issues; in other buildings they had intermittent communication issues between their thermostats and their control modules. During a redesign cycle they changed the RS-485 transceiver that was being used for the 2-wire communication between modules, and reports of intermittent communication errors stopped! What was responsible for this seemingly lucky outcome? Upon closer inspection, the system was found to be failing due to the receiver input hysteresis being too small. The new part they had chosen, the MAX14775, boasts a wide receiver input hysteresis of 250 mV!
Understanding the Basics of Input Hysteresis
A transceiver is the connection to the outside electrical world. Differential signals coming into the A/B pins of an RS-485 transceiver are rarely clean and are far from perfect. The signals are often attenuated, have offsets, and worst of all, they are NOISY. Noise comes from many sources, but the largest contributor is coupled noise from the environment.
An input signal is recognized to be high or low when it crosses a threshold. If there is noise on the line, then the threshold might be crossed back and forth several times when it should only be crossed once. This results in output chatter on the transceiver digital output.
Figure 1: Explanation of the Effect on an Output Signal with and without Input Hysteresis
This is a modern problem that has become more prevalent as transceivers have been designed to operate at higher speeds. Older transceivers with slower responding input stages naturally filtered out noise, preventing chatter. With a faster response inherent with higher speed transceivers, wide hysteresis really helps to reduce the chatter.
How Wide Should the Input Hysteresis be?
The answer really depends on two things:
- The potential attenuation of the signal. This is affected by the type of cable (twisted pair, parallel/flat, or shielded) and distance of the cable, as well as how many nodes are on the multi-drop network.
- The potential minimum signal-to-noise ratio (SNR). This depends on the type of cable, and the environment the system is operating in. Industrial environments are considered harsh environments, where cables will be running near heavy machinery with motors and other electromagnetic interference (EMI) generators.
If the attenuation is large at the node furthest away, and the hysteresis is large, then the furthest away node will miss data (refer to Figure 2(c)). If the SNR is poor, and the hysteresis is too small, then the noise will cause chatter, corrupting the received data (Figure 2(d)).
Figure 2: Overview of How Input Hysteresis Thresholds Can be Affected by Signal Attenuation and Poor Signal-to-Noise Ratio
Figure 2 shows four scenarios where the receiver input hysteresis makes a difference in the quality of communication. Plot A and B show good signal transmission where without hysteresis the bit error rate would be very poor. Plot C shows poor signal transmission that can occur if the hysteresis is too large. And plot D shows a scenario where, due to poor SNR, the signal cannot be recovered even with hysteresis. The application must be considered when choosing the hysteresis for the transceiver.
What is the Answer?
Our recommendation is that transceiver hysteresis should be chosen based on the type of network and the typical environment. Here is an example decision grid.
| Lower Noise Environment | High Noise Environment | |
| Short Cable Distances between Nodes | ~50 mV Hysteresis is Sufficient | 250 mV Input Hysteresis |
| Large Cable Distances between Nodes | 250 mV Input Hysteresis | < 100 mV Input Hysteresis and Consider Shielded Cabling |
Table 1: Example Decision Grid for Selecting Transceiver Hysteresis
Conclusion
Using transceivers with large receiver input hysteresis can be helpful in reducing or eliminating receiver output (RO) chatter. Other filtering techniques (both analog and digital) can be used to address noise-related issues, however, adding hysteresis to the receiver input greatly simplifies the need for additional filtering to remove the chatter.
Wide hysteresis in RS-485 transceivers is a game-changer for robust and reliable communication. By minimizing receiver output chatter, it simplifies system design and enhances performance, making it an essential feature for any industrial application, from HVAC systems to beyond. Embrace the power of wide hysteresis to ensure your network stands strong against noise and distance.
See the blogs in the TranscendingConventionalFieldBus series.