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PICList Thread
'RS485 configuration'
1997\12\04@152000 by Gennady Palitsky

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To all network gurus on a PICLIST,
I have RS485 network of one master and multiple (around 5) remote
controls (with PICs, of coarse). In previous situations I always
daizy-chaned remotes, terminated two far ends and had no problems. Now I
can't do that. So I have star configuration with one master transmitter
connected to at least 5 parallel receivers (with cable length
300-500ft), and all 5 transmitters on remote controls connected to one
master receiver. Slave transmitters have transmit enable, so they are
activated only one at a time. Transceivers are MAX489.
So the question is:
Though I understand that star configuration is not recommended for 485,
what would be the best solution for this situation (from the point of
view of termination, I guess - or whatever).
Will be thankful for any advice.

Gennady Palitsky,
Jefferson Audio Video Systems
spam_OUTgennadypTakeThisOuTspammainlink.net

1997\12\05@044400 by Frans Gunawan

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At 03:16 PM 12/4/97 -0600, you wrote:
>To all network gurus on a PICLIST,
>I have RS485 network of one master and multiple (around 5) remote
>controls (with PICs, of coarse). In previous situations I always
>daizy-chaned remotes, terminated two far ends and had no problems. Now I
>can't do that. So I have star configuration with one master transmitter
>connected to at least 5 parallel receivers (with cable length
>300-500ft), and all 5 transmitters on remote controls connected to one
>master receiver. Slave transmitters have transmit enable, so they are
>activated only one at a time. Transceivers are MAX489.
>So the question is:
>Though I understand that star configuration is not recommended for 485,
>what would be the best solution for this situation (from the point of
>view of termination, I guess - or whatever).
>Will be thankful for any advice.
>
>Gennady Palitsky,
>Jefferson Audio Video Systems
>.....gennadypKILLspamspam@spam@mainlink.net

download the application note about RS485 from http://www.bb-elec.com
they have star configuration for RS485 but need a repeater.
very good app note.
goodluck

http://www.poboxes.com/f
f

1997\12\08@095231 by Lou Calkins

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>To all network gurus on a PICLIST,
>I have RS485 network of one master and multiple (around 5) remote
>controls (with PICs, of coarse). In previous situations I always
>daizy-chaned remotes, terminated two far ends and had no problems. Now I
>can't do that. So I have star configuration with one master transmitter
>connected to at least 5 parallel receivers (with cable length
>300-500ft), and all 5 transmitters on remote controls connected to one
>master receiver. Slave transmitters have transmit enable, so they are
>activated only one at a time. Transceivers are MAX489.
>So the question is:
>Though I understand that star configuration is not recommended for 485,
>what would be the best solution for this situation (from the point of
>view of termination, I guess - or whatever).
>Will be thankful for any advice.

Hello -

A lot of people get worried about terminating 485 lines.  As most are aware,
with unterminated busses, problems occur when transmitted signals are
reflected because the ends of the bus are not terminated properly (i.e.
resistive load at the characteristec impedance of the line).

How high is your baud rate?  I would think if you are running 9600 baud,
terminating the bus is not as critical as if you ran 115kb.  The reflections
are so fast that the received bit might not even be distorted at the mid-bit
sampling position.  Your star situation will require a terminator resistor
at the master and one at the end of each star leg.  There is nothing wrong
with having a bunch of terminator resistors - it just means you cannot drive
as much cable or as many receivers.

I am assuming you have a 2-wire bus.  If you have separate transmit and
receive busses (a 4-wire bus), then the master transmitter only needs a
terminator at the distant ends of the receiver lines.  A terminator is only
needed at the end of the direction of travel of the signal (because if you
terminate it, there is no reflectied pulse returned).  So if your situation
has a 4-wire bus (2 wires for master xmit to all recievers and 2 wires each
for all remotes back to a master receiver) then you only need to terminate
the "end" or destination point of each bus.  That means one resistor for the
master receiver, and one resistor at the end of each star leg of the master
transmitter.

--- Lou

1997\12\08@135701 by Bruce Cannon

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>To all network gurus on a PICLIST,
>I have RS485 network of one master and multiple (around 5) remote
>controls (with PICs, of coarse). In previous situations I always
>daizy-chaned remotes, terminated two far ends and had no problems. Now I
>can't do that. So I have star configuration with one master transmitter
>connected to at least 5 parallel receivers (with cable length
>300-500ft), and all 5 transmitters on remote controls connected to one
>master receiver. Slave transmitters have transmit enable, so they are
>activated only one at a time. Transceivers are MAX489.
>So the question is:
>Though I understand that star configuration is not recommended for 485,
>what would be the best solution for this situation (from the point of
>view of termination, I guess - or whatever).
>Will be thankful for any advice.

Not a guru, but a fellow seeker, and in my research came across B+B
electronics' fabulous tutorial.  They give info on how to decide whether
you need to terminate based on data rate and cable length.  You would need
to know the cable's propogation velocity supplied by mfr.  As others have
mentioned, see

http://www.bb-elec.com/welcome.html

and look for their RS-422/485 Application Note, and check out p.16

Bruce

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