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Topic: RS485 buss connections using USB cables. Yes/no? (Read 515 times) previous topic - next topic

oric_dan

I am thinking of ways to connect a network of nodes together [small Arduino pcbs] using the RS485
buss, and wonder if there are any obvious problems with using standard USB cables for this. The
nodes might be 10-30 feet apart, and only one [the host] might be powered directly, and with all
the others drawing their power off the daisy-chained USB cables.

Each pcb would have 2 small USB receptacles for daisy-chaining, and with the assumption that the
total current load would not be excessively high, ie less than say, 1 Amp.  The raw unregulated
power, approx 6-7VDC, would be passed down the cable, with local v.regs on the pcbs.

I notice USB cables typically have twisted shielded pair for the D-lines. Any obvious problems I'm
not seeing here? It seems the alternative is to jin up some sort of connections using raw dual
twisted-pair cable.

http://www.getusb.info/review-usb-cables-from-tendon/
http://www.accesscomms.com.au/reference/usb.htm

retrolefty

I see no reason they wouldn't work electrically. Do be aware that there is probably large quality differences in specific USB cables, I'm sure they don't all use the same gage wires. 

Lefty


Nick Gammon

When I tried to power my Raspberry Pi from a USB cable I had lying around, I had to chop it in half to minimise the voltage drop, so it would boot. And the cable wasn't that long to start with.

So, if you are expecting large currents to be drawn, I would be cautious.
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics

oric_dan

Hmm, one good, one bad. So far, this is just a conjecture about possibilities. The spec does
seem to indicate currents to 900 mA, so I figured the cables themselves should handle that
without too much loss, so Nick's comment is a little unsettling.
Quote

http://en.wikipedia.org/wiki/Universal_Serial_Bus
Max. voltage   5.00±0.25 V (pre-3.0); 5.00+0.25-0.55 V (USB 3.0)
Max. current   500-900 mA (general); 5 A (charging devices)


Plus the spec is talking about supplying 5V +/- a bit, while I was thinking of sending the
unregulated Vin voltage down the chain. It was just a thought, so you could power a small
network of nearby nodes from only a single battery or wall-wart.

I wonder if the r.Pi is overly sensitive in regards regulation - a lot of circuitry there.

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