cat5 cable for RS485 and power ?

I have to link up about 100 microprocessor based boards together with RS485 lines and connect to a host. I have to provide dc power to those boards too. I was wondering can I use onea cat5 cable for the purpose ? As there are few pairs of wires within a standard cat5 cable, can I use two pair for communication and other for dc power supply? Is there any standard for such usage already ? I appreciate very much your any comments.

Reply to
john
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Reply to
john

i may be wrong but i think seeing some where RS485 handles up to 32 devices on the same line.

Reply to
Jamie

google POE, .....power over ethernet

martin

Opinions are like assholes -- everyone has one

Reply to
martin griffith

Yes, POE.

There are various implemations, but the Max draw seems to be 1A and a 48Vdc power buss on the unused pairs. This has been adopted by various mfg's that produce off the shelf injectors and splitters.

Cheers

Reply to
Martin Riddle

Why use Cat5 cable? I've used 4C18Ga 'Thermostat wire' for almost 30 years in remote energy control systems. Each building has a central 24V power supply and up to 64 energy control panels.each draws a max of 50mA( all 400 series CMOS). We routinely pulled up to 2000 ft of this wire and commuications and power was never a problem. I know CAT5 is cheap, ready made cables,etc. but be aware of the 'configuration' problems you might encounter. Cat 5 usually has only 4C in it, now if you used all 8, you'ld get better reliablility.

Jay

Reply to
j.b. miller

I do something similar, using LVDS at 80+MHz instead of rs485. Each card takes about 300mA at 24V, and this all works with cables up to 20m. You need to watch two things - one is common mode voltage, caused by the current flow. This should not be a problem with rs485. The other issue is grounding - if you ground anything at the remote card, then you will probably get ground loops ...

Another problem I have run into with a system like this is startup - you tend to get interesting waveforms when you put the power on, often with comparitively slow risetimes and strange glitches - all of which sometimes results in chips not initialising properly.

Dave

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Reply to
Dave Garnett

The comm's down CAT-5 obviously isn't a problem, but what current draw are we talking about per board? What distance? I don't know of a 'standard' for this, I've seen several systems do it, differently, but generally the outer two pairs on the RJ-45 are used for the comm's and the others for the secondary purpose. I'd use the white or mate for negative jolts and the color for the positive jolts

Ken

Reply to
Ken Taylor

One problem is that if you want to use RJ45s on the ends, you have to daisy chain cables through your boards, and so wind up putting 200 connectors in series with your last device. This may be, ahem, a bit of a reliability headache. One connector is bad enough already!

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Maybe the cheapest, but not the shittiest. I got some device on ebay, that had a separate power supply, three voltages. The cord from the PS to the unit was the stiffest 5/16" (4 mm) cable I've ever had the misfortune to screw around with, and its connector was a mini-din MOUSE PLUG! It's hard enough to get one of those things lined up in the first place, but attach it to a cord that takes 50 ft.lbs to twirl, and it's a prescription for bent pins.

I ended up tossing the whole thing.

Thanks, Rich

Reply to
Rich Grise

The problem with RJ45 connectors (and all of that ilk) is they are a single-beam contact. This makes them extremely prone to vibration - move the pin away from the contact and an open circuit results.

Compare that with the dual-beam contacts in most connectors - move the pin one direction and one contact is disconnected, but the other contact improves - voila, no problem.

Plus, of course, these are often made by some cheap asian manufacturer using non-existent quality control and for the lowest possible price.

mind you, your mini-din connector sounds diabolical too.

Cheers Terry

Reply to
Terry Given

And lets not forget that RJ45 connectors are the cheapest, shittiest connectors known to man. After all, who really cares if we get a burst of noise in an audio transmission.

I have seen first-hand the results of using them to supply gate drive signals to 100kW half-controlled 3-phase bridges, which turned out to be a bad idea. The hilarious part was they were used partly because they were really cheap (in $50,000 worth of equipment - go figure), but also because the guy who designed them in thought they would be reliable (hey, they are used everywhere in Telco circuits right).

Cheers Terry

Reply to
Terry Given

So what would you recommend as far as connectors for RS485 communications?

Reply to
jd.newsgroups

As usual, it all depends on what the communications are, how important it is they work, what budget available, the environment etc. etc.

Ideally the connector you use will have a nice co-axial connection to the cable shield, and likewise a coaxial connection to the socket. This cures the HF emissions/susceptibility problems caused by shield "tails" (if done properly). The pins will have multiple, spacially distributed contact points.

In practice I often use Phoenix rising-clamp screw terminals and ferrules on the end of the wires, along with some decent connection to the cable shield (eg clamp exposed shield braid onto tinned Cu on pcb). Them metal circular connectors are great, but a pain in pretty much every aspect, especially the wallet. I also make sure my comms protocols incorporate CRC for message validation, and maximise the hamming distance between valid command messages - as opposed to, say, allocating one bit to one task, which is nigh impossible to error-check and has the added "feature" of allowing a single bit error to directly control something, not a good idea if say its the "on" flag for a huge electric saw.

OTOH if you can live with the potential reliability issues and are happy to replace dodgy connectors then RJxx can and do work fine.

Cheers Terry

Reply to
Terry Given

I read in sci.electronics.design that Terry Given wrote (in ) about 'cat5 cable for RS485 and power ?', on Thu, 24 Mar 2005:

The shield should go to the *enclosure*, assuming it is conductive, preferably to the outside surface of it.

The shield is an antenna, and connecting it to the PC board puts all its RF current into the place where it is liable to cause the most grief. Equally, if the PC board 'ground' is hot at V/UHF at the connection point, the shield broadcasts that to the outside world, especially to the antenna measuring the emissions for FCC/EMCD conformity testing.

--
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Reply to
John Woodgate

snipped-for-privacy@gmail.com schrieb:

Since reliability is of concern, I would stick with something you can press into the main cable. The main cable just runs down the line and is being terminated on both end with for example, the typical 120Ohm's. On the microprocessor side, if you can go with DSUB. DSUB is an old but still good connector in my opinion. Easy available and really field proven ...

Have a nice day ...

Markus

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Reply to
Markus Meng

Thanks a lot for your comment, I haven't thought of this point. Is there any reliable replacement for RJ45 plug in terms of size ? Assuming we could afford a little more expensive plug.

Reply to
john

It's pretty common to stick 48V on a pair to provide power. One standard specs 350mA maximum, although you're only supposed to rely on being able to pull 12.95W from the pair. Hence, with one pair, enough of your boards would be limited to ~130mW... if you use two pairs for power, you're up to

260mW per board. (Some of that will go into the losses for the switching regulator you use to get back down to the 5V or 3.3V or whatever that your microcontroller needs.)

Here's some more information:

formatting link
. It's about power over Ethernet, but RS485 will work too.

---Joel

Reply to
Joel Kolstad

Unless these are very low power uP boards, even a few 10s of milliamps per board x 200 boards winds up being a lot of current to carry in even a few 22-26 ga wires. Also, that much load will introduce a humongous amount of digital noise on the supply lines which would need to (a) be filtered out on each board and, worse, (b) tend to corrupt the communications signals.

Norm

Reply to
Norm Dresner

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