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'[EE] Routing 1 of 2 RS-485 inputs to 1 output'
2018\03\10@122824 by Jim Ruxton

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Just curious how others might handle this scenario. I have 2 RS-485
signals. I want to route one of the 2 to a UART pin on a PIC. I am
looking at using 2 MAX485  receivers or something similar. These have RX
enable pins that either output the signal or put the output pin into a
high impedance state. I am wondering about tieing the 2 outputs together
and be sure I only have 1 of the 2 outputs enabled. I am just concerned 
that on power up the outputs may not be in a high impedance state and
will fight each other until my PIC is fired up completely. My other
option is to join the 2 outputs with series resistors.  Since I am going
to have a single optical isolator after the MAX485s I could tie the
outputs together through  current limiting resistors going to the
isolator. Or maybe someone has a better suggestion or knows another
rs-485 IC that does this ie. allows the selection of 1 of multiple
inputs. I did a quick mockup of the 2 scenarios only showing the
relevant bits. I am considering here.
https://drive.google.com/open?id=1im2HMS2sjCmgZup1q0AzfC9YSY-Z1ew2

I would appreciate any thoughts or opinions.

Jim

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2018\03\10@123925 by George Smith

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Jim Ruxton wrote:
> Just curious how others might handle this scenario.

Knowing very little about RS485 chips, I would just use a dual opto-isolator with the output transistors in parallel.

George smith
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2018\03\10@125317 by David Strumpf

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I would consider circuit B as you have shown the use of the RO pins. However, I would recommend additionally adding a 10k resistor on each RE* pin as a pull-up to vcc.  This will ensure that the two RO pins remain in high impedance state during power up.

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2018\03\10@135230 by alan.b.pearce

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Umm, does the enable pin control the digital side or the RS485 side?

I understood it controlled the RS485 side so you could have multidrop communications.

However I haven't looked at MAX485 chips specifically.



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2018\03\10@142947 by David Strumpf

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What I believe you are thinking of is the DE(driver enable) pin.

The RE* (Receiver Enable) pin is like a tri-state chip enable for the RO(receiver output) line. When the RE* pin goes low, the RO pin becomes active as a "TTL" level output. The RO output represents a live monitor of the differential RS-485 level observed on the RS-485 bus.
When the RE* level is high, the RO line is tri-state or Hi-Z.

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2018\03\10@150730 by Manu Abraham

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Hi,

Looking at the MAX485 functional diagram on page 2

http://ledsee.com/datasheet_ledsee/MAX485.pdf

by controlling RE you control the Receiver output

"Receiver Output Enable. RO is enabled when RE is low; RO is high
impedance when RE is high."

If I am not mistaken, It should be as simple as that, if things go well ;-)

RE could be set high by a resistor by default until the PIC is ready
to tango, which would pull down RE as and when necessary.

Cheers,

Manu


On Sat, Mar 10, 2018 at 10:58 PM, Jim Ruxton <spam_OUTjim.ruxtonTakeThisOuTspamgmail.com> wrote:
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2018\03\10@164517 by Isaac M. Bavaresco

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Jim, just use a pull-up or pull-down resistor (depending on the logic
level needed) on the enable pin to ensure that the outputs are disabled
until the PIC is initialized.

Cheers,

Isaac


Em 10/03/2018 14:28, Jim Ruxton escreveu:
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2018\03\10@194415 by Jim Ruxton

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Thanks I was thinking about using the pull-ups but wasn't sure that would
be reliable as the PIC was initializing. I figured it could be dangerous if
the outputs ever met each other in a non high impedance state. This is the
simplest solution however.
Jim

On Mar 10, 2018 4:45 PM, "Isaac M. Bavaresco" <.....isaacbavarescoKILLspamspam@spam@gmail.com>
wrote:

Jim, just use a pull-up or pull-down resistor (depending on the logic
level needed) on the enable pin to ensure that the outputs are disabled
until the PIC is initialized.

Cheers,

Isaac


Em 10/03/2018 14:28, Jim Ruxton escreveu:
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2018\03\10@213413 by Isaac M. Bavaresco

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Jim,

If your code is well behaved, then it is safe to rely on the pull-up (or
pull-down).

PICs don't present glitches on the pins at power-on.

The same is not true for ARMs. Some ARM processors have BOOT ROMs that
try to boot from external memories and actively drive some pins.

ARMs also have weak pull-ups (~100 k) on most of their pins during
reset. That may be good to avoid excessive current consumption until the
firmware initializes the GPIOs but also may interfere with external
circuits.

When I use RS-485 with ARM CPUs, I put a strong (2k2) pull-down on the
TX enable pins, so a unit that is resetting doesn't interfere with the
RS-485 bus.

Cheers,

Isaac


Em 10/03/2018 21:44, Jim Ruxton escreveu:
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