SparkFun According to Pete #54 - How RS-485 Works
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- เผยแพร่เมื่อ 5 มิ.ย. 2024
- 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:
TBS-89-A (specifics on setting up your own RS-485 network): e2e.ti.com/cfs-file/__key/tell...
Renesas tutorial (favors Intersil parts just slightly): www.intersil.com/content/dam/... - วิทยาศาสตร์และเทคโนโลยี
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.
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.
Thanks for the information and the shortcut, have a great day.
Helpful in understanding of basic working.
Thnakt you.
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.
Thank you for the information on the RS-485 which is the basis of XpressNet which is an interface standard within the model railway.
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.
I'm an electrician that just started into industrial automation, everything is modbus which is rs-485. thank you for helping me understand why.
Hey, I'm an senior student electrician and about to enter medical industry. My senior engineer wanted me to search about modbus and im here. It is pretty common yeah
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.
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.
Well noticed.
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.
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.
The first version of RS-422 was issued in 1975. Revision A in December 1978. Revision B in May 1994.
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.
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!
Thanks, and noted. It can be difficult to draw a line on some of these topics as to what needs to be included for a baseline of info.
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.
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
Thanks a lot
Another great informational video, but I think the background music was kind of annoying.
Thanks for the info, but by any chance, do you know how to setup a RS485 wireless link? Thanks for any help.
NICE
I know its like a year ago but I am trying to get my head around RS 485. I have a dome type camera that communicates to a joystick program via RS 485. Continuesly I get protocol error. Now seeing that you mention an earth wire aswell, where does this come into play. Then the second question, does the two main wires have definite A and B postions. Can these be reversed. The way I see it A being - and B +. The protocol is Pelco D and is send via serial ttl port and Max 485.
Thanks for not being a bore, and having a personality. lol.
Data rate can vary with the distance of your cable. How do you determine what the data rate is? How does the receiver know it received 1 bit or more than 1 bit?
Hi Pete, very much like watching your videos. Can you also do one about 1-wire? I have a little greenhouse out back and was researching several ways on how to communicate with my sensors / actuators. I2C has no range, wifi uses a lot of power and is not practical when sensor is battery powered. I came across 1-wire and it seems like an ideal solution; having multiple sensors on a single bus. Nevertheless there aren't that many sensors for it. (why is that?)
Nice video. I take it this is an asynchronous communications protocol? In your closing waveform drawing, it shows you have two time periods without polarity changes so you call them either a 0 or 1. Considering there is no clock signal, is there then a defined data rate for these connections?
Can i use like uart with this standart
That intro music... RIP headphone uses. Yikes
Thank you for your input, we will adjust for future videos!
True that... Ears Ringing..
@@sparkfun Still killing eardrums, other than that nice video.
The link for the TBS-89-A specifics is not correct. It ends up with link not found at the TI e2e site.
My war story is about fighting Sparkfun BOB-9505 the RS-485 breakout board with the mislabled outputs...
We were using RS485 on the Motorola MVME 110 board back around 1989. So what is this 1998 date, is that when it became standardized?
Correction - MVME 106 board
you mentioned a receive mode. How is it selected what node is in which mode?
That's up to the individual node to decide, based on what the protocol which is being used on the bus says should be happening. Nodes will be listening except when they have something to say. Some protocols ensure that (well behaved) nodes can't be transmitting at the same time. Other protocols have strategies (e.g. various wait and retry schemes) for dealing with collisions when two nodes transmit at the same time.
Do you need a terminating resistor on each end or just one end?
You need it both ends of running half duplex but don't need it at the transmitting side if sending data one way only. Generally you'd put both in since it's only a few cents in parts.
Noise reduction and speed are I think the majors of rs-485 Is I am correct?
I'd call that a good summary.
Basically Pete read-out the wikipedia page with a lot of shy and confusion!
Your link to TBS-89-A doesn't work but scratching around at TI resulted in this: www.ti.com/lit/pdf/slla272
I don't know if this is similar to TBS-89-A but it looks like it might be useful.
if you have Way-Back Machine installed, they have an archived copy.
The TBS-89-A link is dead
Heads up, TBS-89-A link is broken
SC means Screen
hey, tech guys, get a sound engineer. holy crap my head is hurting with my reference headphones. im both, hire me. i wouldn't let this 40db difference be posted.
or at least load your vid's audio channel up in any production SW, you will SEE the difference.
Thanks for the input, noted x3
why the music
Uhhh..... I thought this was going to be a quick explanation of how this works. Dude talks for 13 minutes straight. Then says this might be a record short and talks for another minute and a half. haha. I enjoyed listening though and learning. So, basically, it uses voltage differentials to send signals. Got it!
for Advanced learning, not for beginers.
The two camera set up is just annoying.
How other than just having to complain about something is the 2 camera's annoying? just watch and learn something bro
If you're bothered, go look somewhere else. Nobody told you to watch this video.
Worst ink color have been chosen to write on a white board
Yes, but does it love me?
RS-485 can see who you are on the inside, beyond what other hardware interfaces are capable of. They only see you on the surface. But you've got to ask yourself: have you earned its love?
Pete Dokter Awesome