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I have two PCB which has PIC32 as a Controller and two serial port. I tested the 1st PCB and serial port was working fine. I was able to send and receive data. But when I started testing the 2nd PCB, it was only sending the data but was not able to receive any data. I am testing it by using a simple UART code. Both the designs are same. The Tx Rx pins of the controller are connected to a MAX232 IC and from MAX232 IC, a serial connector is connected. I tested for any break in the track but everything seems to be alright. What can be the issue.?

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    \$\begingroup\$ I can't see the schematic. Send it harder. \$\endgroup\$ – user65586 Mar 30 '16 at 16:55
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Well, 1001 reasons. Check the followings:

  • does the failing MAX have power? Are the capacitors properly connected for BOTH MAX (check polarity)?
  • can you clearly measure +8V and -8V (or more) between GND and TX output of both panels? Use a scope and send some data, if you don't have a scope then write a simple on-off-on-off application which turns the output on/off.
  • once you're sure that sending side ok, and you have a valid sender, go to the receiver side.
  • check if the receiver MAX RS232 side pin of the data line show valid voltages (there is no short or disconnect) - so the +8V, -8V what you've just made as the TX side test pattern
  • check if the uart side of the receiver MAX shows the same pattern with 0V, 5V. If yes, then you have a software error. If no, then you have a hardware error.
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  • \$\begingroup\$ What is output voltage at the pin tx and rx when there is data on the pins. \$\endgroup\$ – user007 Mar 31 '16 at 6:28
  • \$\begingroup\$ Why the logic 1 volts is -8v and logic 0 volts is +8v \$\endgroup\$ – user007 Mar 31 '16 at 6:39
  • \$\begingroup\$ The RS232 driver inverts all outputs, see RS232 specifications. So "1" is -12..-8V, "0" is +8..+12V. The receiver side inverts this back. So if you send out an "1", on the line you have -8V, and on the receiver you have "1" (e.g.5V). \$\endgroup\$ – Gee Bee Mar 31 '16 at 11:28

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