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'Optoisolated RS485 input [OT]'
2000\02\28@082705 by Edson Brusque

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Hello,

   I'm using a DS75176 to transform between RS485 to TTL, but now I want to
optoisolate it.

   Someone here have some ideas of how can I make it?

   Thanks,

   Brusque

2000\02\28@132740 by Scott Dattalo

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On Mon, 28 Feb 2000, Edson Brusque wrote:

> Hello,
>
>     I'm using a DS75176 to transform between RS485 to TTL, but now I want to
> optoisolate it.

At what point do you wish to apply the isolation barrier? I'm assuming
you'll want to place the barrier in the TTL interface -right? Otherwise,
if you place it in the 485 interface then you no longer have 485! You only
need three optocouplers in the ttl interface 1 for the receive, 1 for the
tx and 1 for the tx enable. The other issue is powering the isolated
RS485 drivers. But the answers depend on your system. For example, if your
nodes are relatively close, then you may wish to run a 4-wire cable - 2
for power and 2 for the differential data bus. One isolated supply can
provide power for the whole bus (and also provide a convenient location
for the termination). If your nodes are far apart, then you'll most
probably want to run just the differential bus and have separate supplies
for each node (or perhaps clusters of nodes).

There are dozens of optocouplers to choose from. The exact choice depends
on your requirements. If you need fast data rates, then you'll need fast
optocouplers... You may want to place the TX and TXen into a single dip.
But the TX EN doesn't have to be as fast as the TX (unless perhaps you
implement the arbitration scheme BAJ and I were discussing the other day -
then the TX EN can be used to generate a precisely timed arbitration
edge).

>     Someone here have some ideas of how can I make it?

Not without details. What are you trying to accomplish. How many nodes?
How far apart? What data rates? Cost? etc...

Scott

2000\02\28@221628 by Kelly J Kohls

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> I'm using a DS75176 to transform between RS485 to TTL, but now I want
to
> optoisolate it.
>
> Someone here have some ideas of how can I make it?

A bit pricey, but have a look at the Maxim 1480.


Kelly Kohls, N5TLE
Dallas, TX
Email:  spam_OUTkkohlsTakeThisOuTspamjuno.com  OR  .....n5tleKILLspamspam@spam@qsl.net  OR n5tlespamKILLspamarrl.net
Homepage:  http://www.qsl.net/n5tle/

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2000\02\29@021136 by Vasile Surducan

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On 28 Feb 00, at 11:25, Edson Brusque wrote:

> Hello,
>
>     I'm using a DS75176 to transform between RS485 to TTL, but now I want to
> optoisolate it.
>
>     Someone here have some ideas of how can I make it?
>
A good choice is to use H11L1 or PC900 optocouplers with small
led current (under 1mA) and triggers at output.
Vasile

*********************************************
Surducan Vasile, engineer
Institute for Isotopic & Molecular Technology
Cluj-Napoca, Romania
mail: .....vasileKILLspamspam.....l30.itim-cj.ro
web: http://www.geocities.com/vsurducan
*********************************************

2000\02\29@031925 by Graham North

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Remember to experiment with Led current and use the lowest
possible value that reliably turns on the opto.

I have read (and I suppose it is common sense when you think
about it) that opto life is directly proportional to Led current.

I know Leds can take much "abuse" but it will shorten their life.


Graham

2000\02\29@035206 by nick.mitchell

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Don't forget to allow for temperature and ageing effects. What works today with
a minimum current at 23 deg C will not work years from now at 40 deg C or after
the opto has degraded by 50%. Humidity can also cause problems.





Graham North <EraseMEgraham.northspam_OUTspamTakeThisOuTLANDINST.COM>
29/02/2000 08:17


Please respond to pic microcontroller discussion list <PICLISTspamspam_OUTMITVMA.MIT.EDU>

To:   @spam@PICLISTKILLspamspamMITVMA.MIT.EDU
cc:    (bcc: Nick Mitchell/GBPTD/ABB)
Subject:  Re: Optoisolated RS485 input [OT]

Security Level:?         Internal



Remember to experiment with Led current and use the lowest
possible value that reliably turns on the opto.

I have read (and I suppose it is common sense when you think
about it) that opto life is directly proportional to Led current.

I know Leds can take much "abuse" but it will shorten their life.


Graham


'Optoisolated RS485 input [OT]'
2000\03\03@082950 by Edson Brusque
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> At what point do you wish to apply the isolation barrier?

   Between the input connector and the 75176. I don't want all the devices
using the same ground, as this can induce some nasty ground loops.

   Someone here have experience with this? Let's say we have 30 devices
receiving RS485 from a transmiter, can ground loops induce faults,
compromise operations, burn something?

   My devices have this aproximate circuit:

                             to the circuit
                                   ^
 +--------------+     +-------+    |    +-------+     +----------------+
 |   input   (+)|---->|       |    |    |       |---->| link connector |
 | connector (-)|---->| 75176 |----+--->| 75176 |---->|(to next device |
 |           (G)|---->|       |         |       |---->| in the chain)  |
 +--------------+     +-------+         +-------+     +----------------+

   Someone feels the necessity of using optoisolators here? It have to stay
between the input connector and the first 75176.

   Best regards,

   Brusque

2000\03\05@120758 by Edson Brusque

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Hello,

   I've tried to make DMX input circuit like this:


   DMX Direct|  120R    +------+         10K
    XLR pin 3|--\/\/\---|1    5|-------+-/\/\/----VCC
                        | 4N25 |       |
                  +-----|2    4|----+  +----- to 75176 pin 6
   DMX Ground|    |     +------+    |
    XLR pin 1|----+                GND
                  |     +------+    |
                  +-----|2    4|----+  +----- to 75176 pin 7
                        | 4N25 |       |
   DMX Invert|--\/\/\---|1    5|-------+--/\/\/----VCC
    XLR pin 2|  120R    +------+         10K

   But it haven't worked. Maybe the 4n25 is very slow? DMX is 250bps.

   Best regards,

   Brusque

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