Pjotr,
Sorry, I overlooked that single line mentioning the length.
-> Chris: Thanks for digging up the source using transformers for RS-485.
I was also thinking of ISDN transformers. I only remembered
vaguely
that something like that had existed. I agree on the
weaknesses
of that approach.
The RS-485 specification denotes the electrical characteristics, but I
cannot
remember any protocol description within that specification (maybe I
overlooked
this ...).
> > When you are in control of both ends of the communication link, you
> > can decide on how you do it.
I suppose this is the case for your Access Control System. Apart from
design restrictions due to the need of backward compatibility you are free
to use whatever encoding you want when you have total control of your data
link.
Am I right when I suppose that your only freedom of design applies to the
electrical part of the RS-485 interface?
The people responsible for these restrictions in your design should consider
that
the difference in price will be partly consumed by additional measures to
protect
the device and (hopefully no) failures at the test lab with the need for
retesting.
These costs should be considered and it is sometimes nice to make a
calculation
what this means per device. When the number of devices sold is relatively
low,
the result can be surprising.
When the cost calculation within your company does not count the engineering
costs per product any discussion will be difficult ...
Best regards,
Michael
> -----Original Message-----
> From: Piotr [mailto:[email protected]]
> Sent: Friday, February 06, 2004 7:13 PM
> To: EMC-PSTC
> Subject: Re: RS485 and CM choke
>
>
>
> Michael,
>
> I posted exactly the same text. The 1 km is there, but not at
> the beginning.
> An year ago I thought it is important. Now I think no (from EMC point
> of view). The tests are made not on the 1km cable, but on 1m cable.
> The problems made by 1 km are simulated with disturbances inserted
> in that 1m. So for me the disturbance levels (set in
> standards) are what I
> am
> analysing to be sure my device will pass them.
> I agree in real live it is difference if I have 100m and 1km.
> I plan to allow 1km but only in one building. As RS485 must
> be connected
> in one line it is possible to have 1km in one building.
>
> Transformers.
> RS485 has the low frequency elements and even DC. You can use RS485
> transceivers to communicate using transformers (I think) but
> with some curry
> signal modulated with your data and not the standard UARTs in
> microprocessors.
> With enough big trafos (my speed is 115kb/s, and there can be
> 9 zeros or
> ones
> in serie) you can probably transmit standard RS485 data, but
> the problem
> begins
> when the frame is ended, there is (can be) DC (3V) on line,
> than next frame
> begins.
> I suppose the first byte in each frame will be corrupted
> because 0 on trafo
> output corresponds to +3V on input. -3 on input gives -6 on
> output. After
> some
> time the output becomes to be correct. It is if everything is
> close to each
> other.
> Then analyse 1km cable.
>
> Using small trafos - you must detect slopes and not states -
> it becomes to
> be not
> RS485.
>
> Regards
>
> Piotr
>
> ----- Original Message -----
> From: "Nagel, Michael" <[email protected]>
> To: "EMC-PSTC" <[email protected]>
> Sent: Friday, February 06, 2004 1:44 PM
> Subject: RE: RS485 and CM choke
>
>
> >
> > Pjotr,
> >
> > In your original post in sci.engr.electrical.compliance you
> talked of a
> > cable length of up to 1km. This is not negligible and should also be
> > mentioned here.
> >
> > You have to deal with burst, surge and potential
> differences (earth is
> > not the same potential everywhere) as well.
> >
> > When you are in control of both ends of the communication link, you
> > can decide on how you do it.
> >
> > Wanting to avoid the higher cost of optoisolation - did you think of
> > using transformers? I do not have to deal with RS-485 very often, so
> > I leave the maths up to you, but this is comparably cheap.
> >
> > DSL - transformers should be OK for this, maybe E1/T1.
> > Using the center tap for filtering gives you a good common
> mode rejection.
> >
> > I haven't seen this solution for RS-485 yet, but maybe
> there is someone
> > else who did ... On the other hand I see no reason why this
> should not
> > work but I am ready to learn if there is one ...
> >
> > Best regards,
> > Michael
> >
> > Michael Nagel - Senior EMC Engineer
> >
> > FORCE COMPUTERS GmbH Lilienthalstrasse 15
> > A Solectron Company D-85579
> Neubiberg/Muenchen -
> > Germany
> > Tel: +49-89-60814-0 Fax:
> +49-89-60814-376
> >
> > e-mail: [email protected] WWW:
> http://www.forcecomputers.com
> >
> >
> >
> >
> >
> > > -----Original Message-----
> > > From: Piotr [mailto:[email protected]]
> > > Sent: Friday, February 06, 2004 11:40 AM
> > > To: EMC-PSTC
> > > Subject: RS485 and CM choke
> > >
> > >
> > >
> > > Hello,
> > >
> > > I posted my question at sci.engr.electrical.compliance and
> > > was suggested to
> > > post it here.
> > > So here it is.
> > >
> > > I'm trying to find if it is possible to do RS485 without
> > > optoisolation.
> > > EN61000-6-2 and EN50130-4 needs interface to work with common mode
> > > disturbance in 150kHz to 100MHz range of 10V (80% AM
> > > modulation) and 150 Ohm
> > > source impedance. This gives me 18V top signal level. I've
> > > read that it is
> > > good practice to add extra 6dB (measurements inaccuracy
> and element
> > > distortions).
> > > This way I have 36V.
> > > I wont to communicate with 115kb/s (I plan to use IC-s
> with limited
> > > slew-rate), so even high impedance common mode choke should
> > > make no problem.
> > > I assume up to 32 devices and up to 1km line.
> > > I see two solutions:
> > > Solution 1.
> > > CM choke 2x2mH + 2.2nF capacitor at each line to ground. This
> > > reduces 36V to
> > > acceptable values but gives me 1.1nF differential mode.
> > > Can this 1n1 be a problem ?
> > > I can increase L and decrease C. But the higher L the lower
> > > its resonant
> > > frequency and I'll have more problems with attenuating higher
> > > frequencies.
> > > Solution 2.
> > > CM choke 3x5mH with no capacitors. RS485 with third wire
> > > connecting grounds
> > > of communicating devices. I've read somewhere that only 3
> > > wire RS485 is the
> > > right solution. In this third wire I can place 100 Ohm to
> avoid ground
> > > difference currents. This solution seems to be very good. One
> > > coil senses
> > > the CM voltage and subtracts it from signal lines. If it is a
> > > good solution
> > > and RS485 is popular than CM choke manufacturers should have
> > > a large offer
> > > with 3 coil chokes of several mH inductance but they don't.
> > > CM chokes with 3
> > > (and more) wires are widely offered but for higher
> > > frequencies. From that I
> > > think that I am doing somewhere a mistake. But where ?
> > >
> > > Connected with that subject is the question: Can I connect my
> > > circuit ground
> > > with Earth ground via 100 Ohm, and than directly connect
> > > circuit grounds of
> > > communicating devices with this third wire. May be the choke
> > > will be not
> > > needed at all (probably if I reduce my needs from 36V to 18V).
> > > My devices are powered from external 12V power supply
> > > selected by installer
> > > (not me).
> > > I don't know where the problems can arise from in this situation ?
> > >
> > > Thanks in advance for any comments.
> > >
> > > Piotr Galka [email protected]
> > >
> > >
> > >
> > >
> > > -------------------------------------------
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