Series resistor value problem

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#1
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I'm having problems with my transceiver because of the R167-(10k ohms) resistor.

The voltage of DIR_CON is low. (I need it to be low for the transceiver to act receiver)

But measuring the voltage after the resistor it was 1.4V

Because of this the transceiver is acting like a transmitter.

Changing the value of the R167 to 3k ohms, it worked fine.

 

My question is why did the resistor had that effect to the voltage of DIR_CON?

THANK YOUUU!!!

 

This topic has a solution.

keiithyyy

Last Edited: Mon. Jun 17, 2019 - 12:18 AM
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The question is why you had the resistor wired that way in the first place! The resistor should be between GND and re/de to ensure the transceiver defaults to receive when the micro is in reset.

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The input current into DE and /RE are spec'd at a maximum of 50uA. 2 x 50uA x 10k = 1V. Add on the output low voltage of your driving chip and you have your answer.

#1 This forum helps those that help themselves

#2 All grounds are not created equal

#3 How have you proved that your chip is running at xxMHz?

#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

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see the datasheet of SN75LBC184

your circuit is quite different from what recommended in datasheet.

see 11.1 Layout Guidelines , there is also a pull-up res at PIN 1(R)

Majid

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Thank you for the answers!

I didn't design the ckt btw. I just re-used it.

But its my fault not reading more of the datasheet.

keiithyyy

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I didn't design the ckt btw. I just re-used it.

Now you can tell them you have a secret way to improve their intermittent problems that they didn't want to tell you about. 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!