Here we go... This is the first time I've ever used Eagle or drawn a schematic and I can't say that I really know what I'm doing So I'm guessing it's not going to be all that great. I found the drawing process a bit frustrating so I only did one side for now.
Does that look like it would work? Or do I need to connect the unconnected pins to something too?
Graynomad:
RE is active low, on the right-hand chip it's pulled high.
Rob
Aha, thanks for that hint! I thought something might be up with it, with the slash after the name or the "overscore". Learning by doing, I guess
So I need to connect it to ground on the right side one then.
I found this table in the datasheet:
So on the left side, RE can be disconnected, DE to VCC.
On the right side, RE to GND, DE can be disconnected.
Also the small circle on the graphic, on chips that means an active low or inverted signal.
Just some notes about schematic layout.
If you mirror image the remote chip the drawing will be a lot clearer.
When things are tied to power rails it's almost always better (clearer) to have VCC going up and GND going down. Your caps go up.
Most beginners make a real dog's breakfast by running the GND wire to every possible place. You haven't done that which is good, in this case however I would argue that because these chips are physically remote from each other you should draw the GND line between the two to reinforce that they have to have a common GND and that that is formed by a GND wire in the cable.
Also the small circle on the graphic, on chips that means an active low or inverted signal.
Just some notes about schematic layout.
If you mirror image the remote chip the drawing will be a lot clearer.
When things are tied to power rails it's almost always better (clearer) to have VCC going up and GND going down. Your caps go up.
Most beginners make a real dog's breakfast by running the GND wire to every possible place. You haven't done that which is good, in this case however I would argue that because these chips are physically remote from each other you should draw the GND line between the two to reinforce that they have to have a common GND and that that is formed by a GND wire in the cable.
Rob
Actually these don't require a common ground between the local and remote chips, that is one of the advantages of the A/B signals being a true differential signal pair. I'm sure some will disagree with this, but I would say hold off until you actually test such a link using independent power sources at each end, it will continue to work without a common ground wire between the pair.
Your probably right, but I'm kind of a belt and suspenders kind of guy and like to hardware the control signals when I know they are never going to be switched.
Lefty
Welcome to the world of conflicting opinions Nico. This is one of the common dichotomies like Canon/Nikon, Ford/GM etc.
I have experience with RS-485 but Lefty has made a career of using it in very bad environments, so I'll bow to him on this while retaining the right to differ
One more question then. Over here, Icc is specified as 0.375 mA typical, the PDF datasheet I have says max 1.000 mA. So I assume it will be safe to power the chips from the Arduinos 5V if the local Arduino is USB powered?
I don't think it matters here. 485 has better common-mode voltage specs, and I think the chips are easier to find and there's a larger range, but either is good I think.
Yes either can be made to work as long as the driver chip on one end is the same 'flavor' as the receiver chip at the other end.
By the way Wikipedia shows a good three resistor termination method one can use on the receiver end of each path (if using two pair full duplex) which will ensure no false data hits due to noise in case the far end of a link is powered off, it's shown in the first picture showing a series 680 ohm, 120 ohm, 680 ohm resistors wired to the recievers Vcc and ground and A and B signal pins. Good idea.
I finally got around to finishing this up.. And wanted to update the thread with some findings.
The first is that the remote/RX end pin 3/DE must in fact be grounded. I just got garbage if I didn't ground it.
The other is that B on one end must be connected to B on the other end, and A to A. Connecting B to A and vice versa also gave garbage.
It seems to work fine without the resistors too, but I assume that may change with longer distances so I put them there anyway.
Figured this stuff might be good to know.
The chips I'm using are MAX3082E. Amazingly everything worked as expected the first time I connected it, I had to actually disconnect some stuff to make sure it wasn't just the software fooling me This weekend we'll test with a longer cable in between.
Another update.. We put the stuff to the test this weekend. Using 75 meters of cat6 cable between the 2 sides, in an electronically noisy outdoor environment, the chips powered by the Arduinos +5V pin, 2 chips per Arduino/side, no common ground. To top it off temperatures were -15C/5F and there was a blizzard going on.
The result of the test was that everything worked perfectly fine, no trouble whatsoever!