According to Pete: How RS-485 Works

Looking for a beefy, wired data bus where RF won’t cut it? We dig into RS-485 and explore how it works and why it just might be the one you need!

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Spring is upon us! And with spring comes…industrial applications?

RS-485 is a hard-wired bus architecture that offers a good compromise of data rate and distance, and it can get the job done where you can’t otherwise use WiFi or Bluetooth due to its great noise immunity. Capable of point-to-point and multipoint linear networks, RS-485 has been the go-to data bus for industrial apps since the late 1990’s.

In this episode of ATP, we explore what RS-485 actually is and how it works. So sit back with a coffee and a scone, or maybe a bag of Doritos and some Mountain Dew, and learn yerself something about RS-485!

Docs you should check out:

TBS-89-A (specifics on setting up your own RS-485 network – 10/8/18 Link is DEAD!! No other evident replacement, this is left here as a reference)

Renesas tutorial (favors Intersil parts just slightly)


Comments 12 comments

  • FYI, your link to the TBS-89-A PDF (TSB_2D00_89_2D00_A.pdf) is now a broken link.

    Do you have an updated link?

    • Hmmm… it looks as though this is one of those docs that someone has decreed could make them some money. Meaning that the only place I can now find it, you gotta pay…or at least jump through some hoops. I’m really sorry, but I don’t have a good substitute and I’m gonna have to pull the link we’ve got.

      • I see plenty of places where you can buy it. It’s out there if you dig for it. Educational places, Chinese servers. A few places still link to it, it’s just not really easy to find it by the filename.

        It might be helpful to know that Figure 1 is called “general 485 system configuration” and that the quotes matter if you are looking for it.

        Here is a list of all of the 18 figures as well if that might be helpful to make sure you locate the right document.

        FIGURE 1 - GENERAL 485 SYSTEM CONFIGURATION FIGURE 2 - 485 BUS WITH STUB CABLES OFF A MAIN BACKBONE CABLE FIGURE 3 - POINT - TO - POINT CONNECTION FIGURE 4 - ONE DRIVER AND MULTIPLE RECEIVERS (MULTIDROP) FIGURE 5 - ONE RECEIVER AND MULTIPLE DRIVERS FIGURE 6 - STAR CONFIGURATION (NOT RECOMMENDED) FIGURE 7 - 485 BACKPLANE EXAMPLE FIGURE 8 - 485 BUS DC CIRCUIT MODEL FIGURE 9 - MAXIMUM LINE LENGTH VERSUS SIGNALING RATE FIGURE 10 - SIMPLE DIFFERENTIAL TERMINATION FIGURE 11 - CAPACITIVE COUPLING OF THE TERMINATION RESISTOR FIGURE 12 - LOW COMMON - MODE IMPEDANCE TERMINATION FIGURE 13 - MAXIMUM NUMBER OF UNIT LOADS VERSUS COMMON - MODE VOLTAGE AND REQ FIGURE 14 - DETERMINING IF LOAD REPRESENTS A LUMPED CIRCUIT TO THE 485 BUS FIGURE 15. - NOISE MODEL FOR A 485 CIRCUIT FIGURE 16. - IDLE - LINE FAILSAFE BIAS CIRCUIT FIGURE 17. - OPEN - CIRCUIT FAILSAFE BIAS CIRCUIT FIGURE 18. - SHORTED - LINE FAILSAFE BIAS CIRCUIT

  • RS-485 is a differential signal so +/-200mV means that the difference between the A and B signals must be greater than 200mV. So the failsafe biasing network that generates >400mV has more than 200mV of margin.

    The article that helped me the most was “The Art and Science of RS-485” by Bob Perrin published by the Circuit Cellar magazine in July 1999. I can usually get Google to turn up a copy of that in pdf form.

  • You didn’t answer 2 questions that came up in my mind:

    1. Is this a one-way connection so you need two wires to get bidirectional traffic? Or is it half-duplex so directions (can) change? If the latter, is there a master and slaves? Or a round robin protocol or something? I know I could just rtfm…

    2. So basically line A is the positive logic line and B is inverted. What happens when some idiot mixes them up? Do the receiver chips at each end know how to recognize reversed signals (I guess not because you say it’s not a protocol)

    Bonus question: do I understand correctly that “not a protocol” means that the standard doesn’t say how the data is formatted at all? So someone could use startbit-databits-stopbit or Manchester encoding (which would solve question 2)?

    • 1) It’s half-duplex. If you want full, you gotta run another twisted pair. Master/slave is determined by the protocol the user decides to employ. I’d prolly do it that way. 2) What happens? Also determined by the user’s code. It’s not like the inverted data is electrically hazardous, it’s just nonsense. But it’s not like you’re going to have random strangers walking up to your RS-485 bus, talking with a funny accent… Bonus: Correct.

  • Minor question, but when I glanced at the list of According to Pete videos, something caught my eye: No video #54…

    • ‘773! Good to hear from you. 54 was Phase Locked Loops. As for any listing difficulties… dunno.

  • Another great video, Pete!

    I have a question: Was the low level “background music” intentional? (I actually had to turn off the sound briefly to verify that it was in the video, and not coming from some place else around here.)

    The one system I’ve worked with that actually included RS-485 used Renasas processors, though I wasn’t involved in that part of the design. (I did an interesting test circuit for the LiPo battery charger on that system, but that’s off on a different tangent.)

    • Cassy makes me turn off my psychedelic-proggy stuff before recording, so it’s not incidental background. But I’m unwilling to turn the volume up enough to hear whatever it is. I don’t think I can handle my own voice at high volume.

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