cat5 cable for RS485 and power ?

[john]

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.

 

[Ken Taylor]

"john" <johnjohn9191@hotmail.com> wrote in message
news:d1elms$j3h1@imsp212.netvigator.com...

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.

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

 

[martin griffith]

On Fri, 18 Mar 2005 21:39:42 +0800, in sci.electronics.design john
<johnjohn9191@hotmail.com> wrote:

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.
google POE, .....power over ethernet

martin


Opinions are like assholes -- everyone has one

 

[Dave Garnett]

"Martin Riddle" <martinriddle@hotmail.com> wrote in message
news:vjq%d.19453$db6.1656@trndny02...

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.

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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[j.b. miller]

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

 

[john]

john wrote:

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.

 

[Phil Hobbs]

john wrote:

john wrote:

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.


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

 

[Rich Grise]

On Wed, 23 Mar 2005 19:50:55 +1200, Terry Given wrote:

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.

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

 

[Terry Given]

Rich Grise wrote:

On Wed, 23 Mar 2005 19:50:55 +1200, Terry Given wrote:

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.


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

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

 

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

 

[Terry Given]

jd.newsgroups@gmail.com wrote:

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

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

 

[John Woodgate]

I read in sci.electronics.design that Terry Given <my_name@ieee.org>
wrote (in <LKt0e.12404$1S4.1290010@news.xtra.co.nz&gt about 'cat5 cable
for RS485 and power ?', on Thu, 24 Mar 2005:

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).

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.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Markus Meng]

jd.newsgroups@gmail.com schrieb:

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

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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[Terry Given]

Phil Hobbs wrote:

john wrote:



john wrote:

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.


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

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

 

[john]

Phil Hobbs wrote:

john wrote:



john wrote:

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.


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!

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.

Cheers,

Phil Hobbs

 

[Norm Dresner]

john wrote:

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.

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

 

[Joel Kolstad]

"Norm Dresner" <ndrez@att.net> wrote in message
news:2gd2e.20837$cg1.2575@bgtnsc04-news.ops.worldnet.att.net...

john wrote:

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 ?

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: http://www.seattlewireless.net/index.cgi/PoE .
It's about power over Ethernet, but RS485 will work too.

---Joel