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MAX 485 Conections

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Suraj143

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I'm transmitting data from a max485 transmitter to a max485 reciever.
I have some small questions.

1)Is it ok to permanantly tie DE to +5V to make as transmitter? Or after transmitting data it should turn off drivers by pulling DE low?
2)Is it ok to permanantly tie RE/ to 0V to make as a reciever?
3)What about if I use idle polarity resistors on both side as well?
 

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JimB

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1)Is it ok to permanantly tie DE to +5V to make as transmitter? Or after transmitting data it should turn off drivers by pulling DE low?
Yes.

2)Is it ok to permanantly tie RE/ to 0V to make as a reciever?
Yes

3)What about if I use idle polarity resistors on both side as well?
I am not too sure about that, what advantage is there in doing that?
It would be better to put a 120Ohm terminating resistor across the two data lines, as per the diagrams in the Maxim datasheet.

JimB
 

Ian Rogers

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You only put the polarity resistors on one side.... I normally use 150 from B to 5v 150 from A to ground and 150 between A and B.... Just a terminating resistor at A and B at the other side...

I use 150 because it goes quite a way...100+ meters... Also don't forget decoupling caps across the power rails at the other end or the noise is ridiculous...
 

Suraj143

Active Member
You only put the polarity resistors on one side.... I normally use 150 from B to 5v 150 from A to ground and 150 between A and B.... Just a terminating resistor at A and B at the other side...

I use 150 because it goes quite a way...100+ meters... Also don't forget decoupling caps across the power rails at the other end or the noise is ridiculous...
Hi I use Resister from A to +5V & B to GND.Not like yours :(

Yes I have some noise coming into my drivers. Sometimes garbage values coming.I have never used terminating resisters.Do I need them? My cable length is 50-100m.

I read somewhere Transmitter should off when it is not used.But in my case it is always active.So either A or B is high (bus is not high impedance).Does this make noise coupling to it?
 

Ian Rogers

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If you only have two devices, the A to 5v or B to 5v.. Its only to stabilise.. It works both ways round... However A to A and B to B connections... If you connect A to B and B to A, you will get garbage... With 50 ~ 100 m I would definately use the terminating resistor..
 
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