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MAX485, ok with 2 nodes, when 3rd node powered up voltage drop below 2V (SOLVED)
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stan001:
Hi,

I made a MAX485 module with the schematics below...

Everything was working OK when 2 nodes were connected, can Rx or Tx between the master/slave nodes on the Arduino Nano.

- I have tried using 20K pull-up/down at A,B or even no pull-up/down on A,B, same results.

- I tried with 120R termination or 1K R termination, same results.

- Was running at the lowest speed of 9600, same results.

I'm using just very short Cat 5E twisted pair cables during the tests..

I can visually see it is not working as the Rx,Tx LED becomes very dim or not litted at all..

Any idea why does it drop the voltages when 3 or 4th nodes was connected ??


Thank you for reading this..



MAX485-sch



When running on 2 nodes, everything was ok.

MAX485-2nodes

When connected n power up the 3rd node, voltages dropped below 2V

MAX485-3nodes

Here is a picture of my setup with the newly made PCB..

MAX485-Eduboard
capt bullshot:
You have placed a pull-up resistor (R2) on the control input. This puts the output of the MAX485 into active state (transmitting) when nothing is connected. So now you have a transceiver transmitting a static level against your data lines, resulting in the reduced signal level.
Change that resistor to a pull-down resistor and it should work.
stan001:
Thanks a lot, you are spot on..

i removed the 10K pullup and let the Arduino pins control the HIGH/LOW state..

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