So I don't need to connect this 3rd contact at all?
or
should I connect 3rd contact straight to the "-" of the power supply and then I don't need that 3rd line?
maxim datasheet only has 2 signal lines, Vcc and GND.
So confusing. Why did they provide this 3rd contact at all?
PS:
Resistance between input's GND and output's GND is infinite.
Not necessarily, as conditions of use come into play. Is it a shielded cable? How has the shielding been treated at both ends? Is the equipment floating at at least one end? Are there audio cables, HDMI shields to grounded monitors, or other connections to the computer at either end, and have those all been ferreted out?
Since I can't come and inspect your use, how can I determine if you're really 'isolated'? I can't. I prefer to trust dozens of design notes, design guidelines, and other robust documents that have been written, and updated repeatedly, in the last 40 years by teams of very intelligent design engineers providing us with robust usage guidance. Are they all wrong, then? I suspect not. Datasheets NEVER tell the whole story, and a connection diagram on a datasheet such as you have quoted BEGs for an application note to provide context of use.
Because you need Gnd, RS485 is a 3-wire system: D+, D- Gnd ( as is DMX! Every DMX cable has the Gnd pins connected ).
Only to the GND of the master. (if different PSU's would already satisfy you, i am looking right at it since i am debugging something atm. For short distances you can even get by without the terminator)
That's true for the shield, not for the Gnd connection.
You can take a look here.
And please stop confusing the TO.
COM is the commen reference for receiver and sender. Usually its connected to Gnd.
Now i am daring you to open a DMX cable, since they tend to contain just a shielded twisted pair.
Well yes that is the cable that is normally used, where the shield is connected to Master GND
Well that contradicts
So if you guys could sort out together who is right and who is wrong that would leave me to do useful stuff.
Talking about Erroneous info on the internet btw.
There is no light guy who is going to be happy with not just 1 broken fixture but the whole club full of broken fixtures.
Since for the OP it doesn't matter as he is powering both from the same source, and i really can't be bothered with you guys i am leaving it at this.
Your statement from #11 is still false. All my stuff works i can forward WS2812 signal through a cat 5 cable over 25 meters (that is 800KHz signal but with varying duty cycle so comparable with something like 3Mbps ) without sharing GND using individual PSUs with 2 MAX485 transceivers (The receiver terminated with a 120R resistor) So it does work, even in hostile environments like near a 5G tower.
In fact how about you write to MAXIM and request clarification on their diagrams, so you can prove your point.
I'll say it again. Proving it works in one instance doesn't mean it's being used as designed. It means you're getting lucky, that's all, and says nothing about the next use case, or our OP's case.
Frankly, I'd rather he be told to use a GND that might not be necessary in his particular instance, than be told to not use it, and be left wondering why his data comms fail at random hours of the day when some device in his house, or office, turns on. Often, it's modern HVAC, filled with VSD drives.
The internet is full of, "well, they said it wouldn't work, but I got away with it, so they must be wrong". Doesn't prove a thing.
let me know when they add the GND line to the datasheet.
It's in the RS485 specification which for some reason you refuse to believe.
Maybe you should contact the originators of the spec and tell them they are wrong.
Yes i know in the wiki it does state that, but if it's not in the datasheet, then for this particular hardware, it is not required.
But you said there was ??
@vvb333007,
Your topic has become a debate about whether ground is needed for RS485 connection, some say it is, some say not. I've no doubt that the people discussing this know what they are talking about, sometimes experts disagree. My advice to you is to include a wire for the common ground and try with it connected and not connected and see which works best.
If you wish I can split this debate into a separate topic, leaving this one free for any other questions you might have related to your project. If you request it be split then I will split it, if you don't request then I'll assume you are happy for the debate to continue here.
Everyone else, please keep the discussion polite and respectful, I know every one of you debating the common ground issue to be very knowledgeable and I know you are trying your best to help the OP.
Thank you.
So it's not even connected.
The most "common" aliexpress modules for Rs485 are the one you have and the other one with DE and RE pins. The other one doesn't even have common ground pin like yours. So in both cases they decided not to offer common ground.
I personally think there should be common gnd, but I disagree that they "don't work" without.
Hello,
Was reading yours and @jim-p responses in a number of MAX485 threads regarding voltage levels. Sellers at aliexpress says that this breakout board works (can be powered) from 3.3 to 5 volts
Datasheet says MAX485 requires 4.75v minimum. People on internet have mixed opinions but many confirm that it works with 3.3
What should i do in order to use it with my ESP32 (3.3 device)?
Just power my MAX485 from 3.3 source and hope for the best (cable length 22m, STP CAT5 copper, don't expect much noise)
Power it from 5v and place a voltage divider on RX (2 resistors). Feed TX without any level shifting
Power it from 5v and put a current limiting resistor (say 10k) on an RX line?
Power it from 5v and use proper level shifter for both TX and RX
To be honest I would like to avoid using a level shifter as much as possible
There should be no noise on the lines (it is shielded twisted pair + it is all underwater)
What do you think? Can I get away with #1, #2 or #3?
Thanks!
Easy enough to try it out. I'd start with the simplest approach and then build from there until it works well.
