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

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.

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

Helpful in understanding of basic working. Thnakt you.

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.

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.

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.

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

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

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.

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!

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

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.

Thanks for the info, but by any chance, do you know how to setup a RS485 wireless link? Thanks for any help.

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

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.

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.

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.

Thanks a lot

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?

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.

The link for the TBS-89-A specifics is not correct. It ends up with link not found at the TI e2e site.

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.

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?)

I get a 1.5v as the differential voltage... Can you explain why?? Also I have a problem with soil sensors that use rs-485 I tried 3 different ones...

That intro music... RIP headphone uses. Yikes

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?

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

NICE

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?

you mentioned a receive mode. How is it selected what node is in which mode?

The TBS-89-A link is dead

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

Do you need a terminating resistor on each end or just one end?

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.

Noise reduction and speed are I think the majors of rs-485 Is I am correct?

My war story is about fighting Sparkfun BOB-9505 the RS-485 breakout board with the mislabled outputs...

Heads up, TBS-89-A link is broken

Basically Pete read-out the wikipedia page with a lot of shy and confusion!

SC means Screen

why the music

The two camera set up is just annoying.

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.

Worst ink color have been chosen to write on a white board

for Advanced learning, not for beginers.

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!

Yes, but does it love me?