RS-422/485 is a widely known and widely used serial communications interface. You can read about it here. Though you could theoretically translate any type of single-ended digital signal into a differential RS-422/485 signal, systems are often designed with UART as the communication protocol and RS-422/485 as the physical layer. I appreciate UART for its simplicity and versatility, and overall I think that UART combined with RS-422/485 is an excellent communications interface for many applications.
The fundamental component when implementing RS-422/485 is the translator IC. This chip ensures that your typical single-ended digital signals are translated into differential signals that fulfill the requirements of the official TIA/EIA standard. These translators are nothing new, but as with most everything else in life, there is always room for improvement.
The MAX14775E is a new RS-422/485 transceiver from Maxim. It provides the typical functionality that you would expect from any RS-422/485 translator, but it also has features that help your system to maintain reliable communication in noisy, high-voltage environments.
A typical RS-485 network with the MAX14775E, which is a half-duplex transceiver.
Caution: High Voltage
Embedded systems operating on 3.3 V or 5 V don’t pose much risk to an RS-422/485 interface. The TIA/EIA-485 standard specifies an acceptable common-mode voltage range of –7 V to 12 V, so even if the common-mode voltage of a transmitted signal is, say, 5 V higher than the ground potential of the receiver, the two devices will still communicate successfully. However, not all systems are limited to such benign voltages. Industrial environments comes to mind, but relatively high voltages can also appear in robotics, aerospace, and medical applications.
The MAX14775E goes far beyond the TIA/EIA standard when it comes to high-voltage tolerance. The driver outputs and receiver inputs can withstand fault voltages of ±65 V; furthermore, the device detects fault conditions and disables driver outputs for a specified period of time following the fault event. In addition to this fault tolerance, the MAX14775E has a common-mode voltage range of –25 V to 25 V, as opposed to the –7 V to 12 V range required for TIA/EIA-485 and the –7 V to 7 V range for TIA/EIA-422.
Coping with Noise
Noise is everywhere, but it tends be more problematic in high-voltage environments. With higher voltages and currents around, radiated fields are stronger, and low-voltage signals are more seriously affected by the resulting interference.
Ordinary digital inputs are inherently sensitive to noise: if a single input threshold determines whether the output is logic low or logic high, small variations centered around that threshold will lead to spurious output transitions.
Without hysteresis, noisy input = unpredictable output.
The standard solution here is “hysteresis,” which refers to the difference between the rising-edge input threshold and the falling-edge input threshold. I had some difficulty determining if any amount of hysteresis is required by the TIA/EIA-422 or -485 standard (I think not)—but, in any event, it seems that many translator ICs provide at least 20 to 50 mV. The MAX14775E, in contrast, has typical input hysteresis of 250 mV, which means that a rising or falling input edge can include some seriously high-amplitude noise and yet not cause a spurious output transition.
High (and Low) Speed
Despite RS-422/485’s relatively simple component and cabling requirements, it is not limited to the 1980s-style baud rates that come to mind when we think of UART interfaces in general. This is especially true when the transmitter and receiver are in close proximity because maximum data rate decreases as cable length increases.
Despite the focus on reliability and robustness, the MAX14775E’s minimum maximum data rate (PDF) is 20 Mbps. (No, “minimum maximum” is not a typo. The idea here is that the worst-case maximum data rate is 20 Mbps.) You can find faster parts; a quick Digi-Key search shows RS-485 transceivers advertising speeds as high as 100 Mbps. But I think 20 Mbps is pretty good considering the other features offered by the MAX14775E, and furthermore, 20 Mbps is perfectly adequate for many applications.
The timing relationship between the single-ended input and the differential output.
One little complication is that we don’t always want high speed. Sometimes we are more worried about EMI. High data rates require fast transitions, and fast transitions require steep edges, and steep edges generate more EMI.
Thus, in some cases we would rather have a driver that intentionally limits the slew rate of the output signal; this would reduce the maximum data rate, but it would also reduce radiated EMI. The folks at Maxim understand this problem (undoubtedly much better than I do), and thus they offer also the MAX14776E. This is the same part except that the output is slew-rate limited and, consequently, the minimum maximum data rate is 500 kbps.
Have you designed any RS-422/485 systems lately? Feel free to share your experiences or component choices.