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Minimum bit rate for this RS485 circuit <?>

dxpwny

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Am I looking at this circuit right ? For RS485 communication with this circuit, wouldn't the use of R5/C2 imply that there is a minimum bit rate that can be supported ? After the LOW start bit, if the next bit is a 1, and the bit rate is not fast enough, C2 charges up to the point where is flips the level applied to the MAX3485 DE input.

e3059cc0103247bdba18129c99e667e5.png
 

KlausST

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Hi,

This is an automatic "driver enable" circuit.
It enables the driver with sending the first LOW bit (START bit) and releases the driver after being HIGH for some time.
The driver is at least long enough ON to drive the bus HIGH..
So a LOW has no minimum or maximum restriction woth this circuit.
And since the bus lines are biased "HIGH" they will remain HIGH when the driver is OFF.
So there is no restriction in HIGH timing, too.

So I expect it to work. But the HIGH - when the driver is OFF - is weak somehow. Maybe just 350mV. So avoid much noise.

Klaus
 

FvM

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MAX3485 has up to 0.2V input threshold, but the resistors are only generating 35 mV bias. Design definitely flawed.
 

KlausST

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Hi,

True...the ratio is about 1:100 and not 1:10 as I mistakenly thought.

Klaus
 

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