SparkFun According to Pete #54 - How RS-485 Works

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 it’s great noise immunity. Capable of point-to-point and multipoint linear networks, RS-485 has been the goto 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:

[NathanSeeley]

I'm an electrician that just started into industrial automation, everything is modbus which is rs-485. thank you for helping me understand why.

[davidmoran]

Thank you for mentioning the ground, many people hear the terms 2 wire or 4 wire RS-485 and think that is all there is, neither the + nor the - is a return for the other, they both return to ground so it is either 2 wire or 4 wire plus ground for proper operation. We transferred 485 as well as other hardware protocols, with the ability to translate on any drop, many miles over fiber optics and even made counter rotating rings, which gave the same functionality as spanning tree protocol does for Ethernet.

[naikrovek]

There's a lot of knowledge in these videos. I wish it was a bit more organized. That said, I always learn a lot, and am thankful to have them available to me at all.

[davidbowen]

Mr Fix It Dave This presentation was exactly what I need to get the basic understanding I need for a remote reading electrical meter system.

[venusbilandi]

when A goes Low and B goes High, it's a "0" and when B goes High and A goes Low it's a "1".
In both the cases A is Low and B is High.

[SteveJonespilot]

Cool video. I was surprised to learn RS485 was only adopted in the late 90s. Back in the mid to late 80s I worked for the IT group of a large national grocery chain and we used RS-485 to communicate from the store's PC in the office (IBM AT 6mhz.. just for reference!) out to a timeclock. In the late 80s, we added to that link a long cable back to the deli scales. We would use the RS485 to program pricing information into the scales (how much does that swiss cheese cost..). Some larger stores also had multiple time clocks. You had me questioning my memory of it being RS-485, and I thought maybe it was really RS-422, but since both the deli scale and the timeclock had to communicate back, it was really "bi directional" between the PC and the clocks and scales (the PC was involved in every communication, but ONE of the other devices had to talk back in every case). It looks like RS-422 wasn't open collector, so you truly could only have one sender, so we MUST have been using RS-485 as I remember it, right? The WIKI page says RS485 first came on the scene in about '83 even though it wasn't formalized.

[mohamadmansoor]

Thanks for the information and the shortcut, have a great day.

[lgrfbs]

Thank you for the information on the RS-485 which is the basis of XpressNet which is an interface standard within the model railway.

[pantuflip]

Great video, I am from Colombia, South America. I love Ethernet, Devicenet, and Controlnet, but RS-485 it the best when you are at the field, combined with Modbus, and long distances.

[Amy_Dunn]

That intro music... RIP headphone uses. Yikes

[MikeBramm]

Another great informational video, but I think the background music was kind of annoying.

[hansmortensen]

Just a note: the TBS-89-A link is no longer working, but if you have Way-Back Machine installed, they have an archived copy.

[hardbrocklife]

Thanks for not being a bore, and having a personality. lol.

[mrsvcd]

Some notes from TSB-89-A 1) if you combine Figure 4 & 5 you get what RS422 looks like. RS422 is a one master bus with possible full duplex, while RS485 is multi master but only half duplex.
2) in Figure 16 it is shown a bias circuit.
3) in figure 8 it is implied that the differential between A & B needs to be 200mV to flip the output from 0 to 1 and vice versa this is also stated in "4.3 Noise budgeting", so the total relative potential movement is 400mV at minimum.

Also RS422 is a nice way to get you RS232 line extended further than 15m (according to Wikipedia) without the need to change protocol or specialized wires.

[pgdelhiganesh]

Helpful in understanding of basic working.
Thnakt you.

[randomdebris]

The first version of RS-422 was issued in 1975. Revision A in December 1978. Revision B in May 1994.

[djlorenz]

I'm working with RS-485 for a long time now for both my work (PROFIBUS communication) and passion (DMX Lighting) and more or less that's it. Be careful at terminations when going high speed and/or long distance.

[qzorn]

interesting, do you have a video using the saleae logic analyzer testing modbus and testing rs485 to verify the results for testing time-out, etc. 🙄 thanks

[johnmccormick]

Great Video ... Thanks for the links! I agree with prior comment that the intro volume is a killer ... My 2 cents worth is that you could've mentioned shield noise and shield currents ... but that is another topic in and of itself. It does effect RS-485 installed systems tho. Thanks Again!

[KennethFinnegan]

TSB-89-A has about the same idle fail-safe bias network (figure 16 - 620 + 130 + 620 ohms) and also says this gives you 206mV-271mV DC bias, which makes me suspect that they might be defining DC bias as from the middle between the two lines, so while one line is +237mV from the center, the second is -237mV from the center, where you're thinking of DC bias as the whole voltage between the two lines. I don't have a copy of TIA-485 itself to dig into if they clearly defined the DC bias concept there as one way or the other.