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'[EE]: RS-485 Chips'
2000\09\27@112232 by Paul

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Can anyone suggest some RS-485 chips

Does anyone know what they actually do (l think they drive the line with current rather than voltage?) and can they be emulated with transistors or the like ? If anyone has any circuits of line drivers for this application can you let me know please.

Also need a decent australian supplier and prices (rough)...

thank you

Paul Drummond
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2000\09\27@125435 by David Kott

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> Can anyone suggest some RS-485 chips

MAX485 for half duplex, MAX489 for full duplex.

> Does anyone know what they actually do (l think they drive the line with
current rather than voltage?)
> and can they be emulated with transistors or the like ? If anyone has any
circuits of line drivers for this
> application can you let me know please.

If you want example circuits implimenting the above, see their data sheets
at:
http://pdfserv.maxim-ic.com/arpdf/1112.pdf

Erm... they aren't "current drivers" like I think you may be thinking.  It
is a ruggedized voltage controlled bus.  You may be thinking of a "4-20
current loop" or just "current loop", which is a completely different beast.
You may wish to read National Semiconductor's application notes on RS-485.
I have found them quite illuminating.

http://www.national.com/apnotes/RS-485.html

You could certainly emulate RS-485 compliant transmitters with a pair of
FETs.  The receiver would be more difficult, I imagine.  One might start
with a comparator and work on from there.
The nice thing about 485 IC's is how relatively rugged they are.

> Also need a decent australian supplier and prices (rough)...

Sorry, can't help you there.  You might try looking at:

http://www.findchips.com

and

http://www.partminer.com

for a program that will search suppliers and distributers for stock given a
particular part number or description string.

-d

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2000\09\27@132318 by Ricardo Seixas

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>Can anyone suggest some RS-485 chips

MAX485,LT485,SN75176...
I use the SN75176 because here it's the cheapest one.

Take a look at National,Maxim and Linear websites there's a lot of info out
there
I suggest National's app-note 1057 ("Ten ways to bulletproof RS-485
Interfaces") http://www.national.com


>Does anyone know what they actually do (l think they drive the line with
>current rather than voltage?)

Multidrop serial interfaces.
Diferential voltage.

>  and can they be emulated with transistors or the like ?

Why bother? these things are cheap.

>If anyone has any circuits of line drivers for this application can you
>let me know please.

On the datasheets there's some examples.
look also on http://www.lvr.com and http://www.rs485.com


>Also need a decent australian supplier and prices (rough)...

Sorry mate, I'm on the other side of the globe <G>

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2000\09\27@142023 by Oliver Broad

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I'd say there's no one device for every application, since there is a broad
spectrum of uses. Speeds go from typical asynch speeds to 10M or higher.
Half duplex is very common and there is a choice of 8 pin devices with the
same pinout. You might look at your sources to see what is readily
available?

The driver has to drive into 60 ohms as a long twisted pair being used at
high speed would have a 120 ohm terminating resistor at each end. Typical
serial rates are low enough that operation without terminations can be
achieved over short distances using slew-rate-limited outputs.

The reciever is essentially a comparator. The margin is fairly narrow, so a
balanced input is  essential. Some recievers have added value by going into
a guaranteed state when the line is not being driven (some of the Maxim
range) which is invaluable when using asynch serial, since a false
transition may be mistaken for a start bit, causing the real start bit to be
missed.

Oliver.

{Original Message removed}

2000\09\27@152614 by David Kott

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<snip>
> The reciever is essentially a comparator. The margin is fairly narrow, so
a
> balanced input is  essential. Some recievers have added value by going
into
> a guaranteed state when the line is not being driven (some of the Maxim
> range) which is invaluable when using asynch serial, since a false
> transition may be mistaken for a start bit, causing the real start bit to
be
> missed.

National Semi also makes a Dominant Mode transceiver.  This allows you to
"fail-safe bias" a 485ish bus.  This particular chip is targeted toward
class 2 vehicle communication;  j1708 and the like.

http://www.national.com/pf/DS/DS36277.html

-d

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2000\09\27@203536 by mthorley

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Try the Maxim web site.  Like the MAX232 they also make a MAX485.  The RS485
protocol allows for multiple nodes on the one bus, a bit like the old
10BASE2 or coax ethernet.  It is common in industrial applications that use
PLC and the likes.  Very handy.

Try http://www.maxim-ic.com and search for max485, you will find lots there.

Mark



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Brisbane, Queensland, Australia.

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2000\09\28@052954 by rubenj

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Don't forget the ground (common) wire (3rd wire for half duplex or
5th wire for full duplex). Nationals app notes tell You ll about
them. I have found the app note '10 ways to bulletproof Your RS485
networ' (or something) to be very usefull.

I allways add protection components to my RS232/485 ports in the
following manner:

Port pin - 3 terminal LCL network - 20 ohm resistor - tranzorb to gnd
(and tranzorb to the other line for 485 networks) - 20 ohm resistor -
IC pin.

Note that the tranzorbs have fairly high capacitance which could
reduce maximum baudrate, but I have had good results with up to
115200bps. Normally don't go higher than 9600, though.

Date sent:              Thu, 28 Sep 2000 02:22:19 +1000
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From:                   Paul <KILLspamgeniesysKILLspamspamALPHALINK.COM.AU>
Subject:                [EE]: RS-485 Chips
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2000\09\28@193704 by Brian Kraut

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www.maxim-ic.com

Paul wrote:

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