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'[EE]: 120 feet bidirectional commnucation'
2002\02\21@162207 by Andre Abelian

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Hi to all,

Soon my electronic gate will be ready witch is
About 120 feet far from my desk and I need to
Build bidirectional communication with whole
House there are doors and switches I need to know
The status of it. My favorite interface is I2C but the problem
Is that I2C is not good for 120 feet distance I guess my
Choice is RS485 differential phase but what I do not know is
By using 4 wires 2 for receive and 2 for transmit can I communicate
with all 35 sensors ?. So rs485 works like rs232 I do not see any way
of sharing from same bus I need to make communication like I2c but
for 120 feet long any help will highly appreciated.

Andre

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2002\02\21@163350 by Douglas Butler

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If you use 4 wires it is RS422.  RS485 uses 2 wires, but you can only
communicate in one direction at a time.  For your use I think RS485 (2
wires) should work fine as long as you have one master and a number of
slaves.  The slaves only speak when spoken to by the master.  If the
master wants to know if the gate is open, it asks.  Basically the master
polls every slave periodically to see if they have info.  But the master
can issue commands any time he wants as long as a slave is not replying
to his previous command.

Sherpa Doug

> {Original Message removed}

2002\02\21@163848 by Orbit Communications

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You can run RS485 with 4-wires for full duplex operation.

Best Regards

David Huisman
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----- Original Message -----
From: "Douglas Butler" <spam_OUTdbutlerTakeThisOuTspamIMETRIX.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Friday, February 22, 2002 8:26 AM
Subject: Re: [EE]: 120 feet bidirectional commnucation


{Quote hidden}

> > {Original Message removed}

2002\02\21@173553 by Kirk Lovewell

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Andre,

You can attach multiple RS485 trancievers to the same
RS485 bus.  The trancievers go into a high impedence
state when they are not transmitting.

You probably can get by with RS485 (2 wires) rather
than RS422 (4 wires) unless you really need
full-duplex operation.

See link below for a good reference document on RS485
and RS422:

http://www.bb-elec.com/tech_articles/rs422_485_app_note/table_of_contents.asp

Kirk

> Hi to all,
>
> Soon my electronic gate will be ready witch is
> About 120 feet far from my desk and I need to
> Build bidirectional communication with whole
> House there are doors and switches I need to know
> The status of it. My favorite interface is I2C but
the problem
> Is that I2C is not good for 120 feet distance I
guess my
> Choice is RS485 differential phase but what I do not
know is
> By using 4 wires 2 for receive and 2 for transmit
can I communicate
> with all 35 sensors ?. So rs485 works like rs232 I
do not see any way
> of sharing from same bus I need to make
communication like I2c but
> for 120 feet long any help will highly appreciated.
>
> Andre

__________________________________________________
Do You Yahoo!?
Yahoo! Sports - Coverage of the 2002 Olympic Games
http://sports.yahoo.com

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2002\02\22@092625 by Olin Lathrop

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> Soon my electronic gate will be ready witch is
> About 120 feet far from my desk and I need to
> Build bidirectional communication with whole
> House there are doors and switches I need to know
> The status of it. My favorite interface is I2C but the problem
> Is that I2C is not good for 120 feet distance I guess my
> Choice is RS485 differential phase but what I do not know is
> By using 4 wires 2 for receive and 2 for transmit can I communicate
> with all 35 sensors ?. So rs485 works like rs232 I do not see any way
> of sharing from same bus I need to make communication like I2c but
> for 120 feet long any help will highly appreciated.

RS-485 is an electrical spec only and leaves a lot to the individual
implementation.  Typically you use the differential lines to carry *one*
half-duplex signal.  All the transmitters on the bus cooperate using a
higher level protocol to decide which one of them is allowed to transmit at
a time.

Another common implementation is to run two differential pairs when there is
a "main controller".  The main controller is always the sender on one pair,
and different slaves respond on the other pair as the main controller asks
them to.  This scheme maps well to hardware UARTs because the send and
receive for each unit on the bus can be hard wired to one of the pairs.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamKILLspamembedinc.com, http://www.embedinc.com

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2002\02\22@133309 by engelec

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Sherpa,

Thanks for your replay,
I have max1482 chips that I can use.
A few things are not clear for me lets assume
I got 30 slaves and 1 master if 2 slaves try to
Communicate with master at the same time then what will happen?
I2C is kind of different it will wait until the bus gets free
Do you know there rs485 mostly are used ? is there any chip to make
I2C work for long distance ?

Thanks again

Andre


>>>>>>>>>>>>>>>>>>>>>




If you use 4 wires it is RS422.  RS485 uses 2 wires, but you can only
communicate in one direction at a time.  For your use I think RS485 (2
wires) should work fine as long as you have one master and a number of
slaves.  The slaves only speak when spoken to by the master.  If the
master wants to know if the gate is open, it asks.  Basically the master
polls every slave periodically to see if they have info.  But the master
can issue commands any time he wants as long as a slave is not replying
to his previous command.

Sherpa Doug

> {Original Message removed}

2002\02\22@135440 by Byron A Jeff

face picon face
On Fri, Feb 22, 2002 at 10:31:32PM -0800, andre abelian wrote:
> Sherpa,
>
> Thanks for your replay,
> I have max1482 chips that I can use.
> A few things are not clear for me lets assume
> I got 30 slaves and 1 master if 2 slaves try to
> Communicate with master at the same time then what will happen?

That doesn't happen by definition. The primary node directs the secondaries
as to when they may communicate. So if one secondary has been granted
access to the line, the others will not communcate on that line until directed
to do so.

Also be very very aware that EIA485 (RS means recommended standard, EIA is
a actual standard) doesn't have any data link layer, so any communications
protocol you want to use you'll have to implement yourself.

BAJ

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2002\02\22@141917 by Martin Peach

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> On Fri, Feb 22, 2002 at 10:31:32PM -0800, andre abelian wrote:
> > Sherpa,
> >
> > Thanks for your replay,
> > I have max1482 chips that I can use.
> > A few things are not clear for me lets assume
> > I got 30 slaves and 1 master if 2 slaves try to
> > Communicate with master at the same time then what will happen?
>
> That doesn't happen by definition. The primary node directs the
secondaries
> as to when they may communicate. So if one secondary has been granted
> access to the line, the others will not communcate on that line until
directed
> to do so.

By who's definition? There is nothing in the RS-485 spec that prevents that.
The spec says that multiple transmitters will not destroy each other by
attempting to send at the same time, it does not say you can't do it. It's
up to your implementation to watch out for that happening, usually by
monitoring the output and comparing with the input, then retrying after a
random delay, as in Ethernet collision detection or else by assigning a
transmit window to each node.

/\/\/\/*=Martin

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2002\02\22@152757 by Byron A Jeff

face picon face
On Fri, Feb 22, 2002 at 02:18:06PM -0500, Martin Peach wrote:
{Quote hidden}

That's why I included the second paragraph in my post. To quote:
-----
Also be very very aware that EIA485 (RS means recommended standard, EIA is
a actual standard) doesn't have any data link layer, so any communications
protocol you want to use you'll have to implement yourself.
-----

Since the primary/secondary relationship will have to be implemented in
software, it can (and in fact must), enforce those rules.

BAJ

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2002\02\23@060719 by Olin Lathrop

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> I got 30 slaves and 1 master if 2 slaves try to
> Communicate with master at the same time then what will happen?

This will cause a runaway nuclear reaction in the driver chips and all life
as we know it will cease to exist.  Seriously, the data will all be mush and
none of it will get thru.  You have to guarantee this won't happen with the
higher levels of the protocol.  For example, a slave only sends when
requested to do so by the master, and the master makes sure only one slave
at a time is supposed to send.  You still have to deal with things screwing
up and getting out of sync.  I've done this on RS-485 by wrapping everything
into checksummed packets, timeout rules, etc.  I had such a system with an
embedded Pentium master controlling 15 PIC slaves.  Worked very nicely.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, .....olinKILLspamspam.....embedinc.com, http://www.embedinc.com

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2002\02\23@092917 by Edson Brusque

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Hello Andre, Byron et all,

Andre says:
>>I2C is kind of different it will wait until the bus gets free
>>Do you know there rs485 mostly are used ? is there any chip to make
>>I2C work for long distance ?

Byron says:
> Also be very very aware that EIA485 (RS means recommended standard, EIA is
> a actual standard) doesn't have any data link layer, so any communications
> protocol you want to use you'll have to implement yourself.

   actually that means you *can* use I2C (or better, a variation of it) to
implement long distance communication. It's not that dificult and requires 4
wires (plus ground) for the communication.

   Just use 75176's (or other 485 driver/receiver) to carry the I2C
signals. The clock will always walk from the master to the slaves. The data
line is bidirectional, so you'll have to tie the I2C-DATA signal on the PIC
to the RX and TX pins on the 75176 and use another PIC pin to select 75176
TX/RX.

   Best regards,

   Brusque

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