From: "Chris Wells" <[email protected]>
To: "'emc-pstc'" <[email protected]>
Cc: "Piotr" <[email protected]>
Sent: Saturday, February 07, 2004 3:06 PM
Subject: Re: RS485 and CM choke
> Are you avoiding Optoisolation (and associated DC:DC) due to cost,
space
> both?
Due to cost.
> There are few cables available that have the 3rd wire integrated
without
> having to use a 3rd wire off a second data pair for this purpose.
> What most industrial RS485 users do is use the shield as the return.
> And so have a driven shield.
> The shield would be AC referenced to ground at each node
(.0047 -.0022 uF Y2
> to ground and 1M bleed off resistor)
> I expect this will raise the hair on the backs of the shield purists
here
> but I challange you all to come up with a solution that would allow
the
> customer to use a single twisted pair cable with shield that would
support
> the needs of RS485 and various code application issues.
I supposed the most popular is the Ethernet cable 4 pairs or 2 pairs
no shield. I thought of both wires from second pair used as 3rd wire.
As no shield I was trying to find solution with my CM chokes. Now I am
thinking rather (not sure yet) of shielded cable, and second pair as
third wire, a shield may be AC referenced to ground, blocked at some
level by transils. All happy ? (I suppose not)
> Distance
> At 115Kbaud - 1KM is a long distance
I will consider reducing my needs to 500m.
> You will have to consider the RC loading of the cable (~35pf/M?),
loading of
> the transient protectors and the loading of the transcievers
themselves.
> Check out the 1/8 loading transcievers like this one from Maxim
> http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1522/ln/en
>
> You will want to specify a low capacitance cable for this distance
and limit
> the number of nodes on the link.
> If you use 1/8 loading transcievers and limit system maximum number
to 32
> you may be able to go the distance at this baud rate.
> However you have to consider some transient protection.
If the cable is well terminated (R=Zo) than its pF/m seems be
unimportant. The transient protectors loading I am afraid but in the
sense of Zo discontinues and reflections, but not in the sense that it
adds to transceiver loading which I think is the DC problem and not
AC. Before end decisions I will check selected transient protector
loading in practice (I have here 1200m of 2 wire cable). I don't know
how to calculate its effect on transmission line. I hope that using
transceivers with it data rate reduced helps a lot. I can estimate the
time needed to load theirs capacitance and compare with my bit time.
> About 4 ohms on each data line and 100 ohm on return to terminal
helps a
> lot.
I supposed the 600W TVS is OK (if 18V) for transmission lines (1kV
Surge with 42 Ohm source resistance (=25A) ) and need no current
limiting resistors.
If I use shielded cable my needs goes down.
> It will effect your impedance matching but the protection is worth
the
> compromise.
I don't believe in impedance matching of RS485 transceiver. If it is
receiver it don't loads line - OK. If it is transmitter I think it has
lo impedance. The signal travels towards both ends and hopefully don't
reflects. As only one transmitter can be at any time it is OK. If
there were two each of them would make reflections of signal send by
the other one.
> You must use a low capacitance TVS or your baud rate will not work.
> Check out biderctional 500/600 joule TVS with built in series diode
that
> pulls capacitance way down
> http://www.microsemi.com/datasheets/MSC1701.PDF
> http://www.microsemi.com/datasheets/srlc05.pdf
> http://www.microsemi.com/datasheets/sd26a.pdf
Thanks, I was waiting somebody points it. I couldn't find in my notes
where to find lo capacitance TVS. I think series diode gives not a lot
until the main TVS capacitance is loaded. So beginnings of
transmissions can be corrupted. But I have never checked this my
opinion.
> You said you need to deal with CM noise up to 100MHz
> You could simply add two ferrite beads on the data lines along with
the
> series resistance.
I know it. I see the main problem at low frequency band end.
> Check out http://www.steward.com/ and look for beads to give you ~
50-90
> ohms at 100MHz.
> Opt for current rating that won't saturate quickly (1 Amp).
Why 1A. I was thinking about 1kOhm/100MHz and about 120mA DC current
rating.
Piotr
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