AnsweredAssumed Answered

AD2587E drawing too much power when transmitting (PSU goes to 2V)

Question asked by MindFlayer on Oct 1, 2012
Latest reply on Oct 8, 2012 by MindFlayer

Hello,

PFA the schematic used to send data from uC to outside via 485 bus. Please note the following:

 

The Problem:

  1. When transmitting the DE pin pulls the supply down to 2V even with a 1000uF cap on VCC (pin 8 - 9, not shown on sch). Mostly the data gets through (as evidenced by the multiple products working with the same sch).

 

Notes:

  1. I isolated the RE (pulled up) and DE (to the uC pin) and put the device in transmit only mode and still the symptoms persisted. In receive only mode there was no problem
  2. If I d/c the DE pin from the AD2587E end everything is fine (no transmission though)
  3. If I connect the DE to the supply directly it pulls the supply down (LM7805)
  4. There is no terminating resistor 120 ohm as on sch.
  5. Yes, there is no isolation it is by design.

 

Let me know if you have any questions.

 

Thanks!

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