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MAX3221: Communication Errors

Sam
Prodigy 10 points
Part Number: MAX3221

 I use four independent RS232 Tranceiver on a new pcb design and have a lot of errors during transmit. The signal on the RS232 side looks really bad. I  increased the value from C8 and C10 to 300nF but it's still not working.I have the same issue on on all RS232 tranceivers.

Channel 1 RS232 Tx signal (R10)

Channel 2 UART Tx signal (R12)

Tx osciloscope

Schematic

  • +1
    Guru 317260 points

    All capacitors must be as near the chip as possible. Please show the board layout.

    You can also try increasing C12 and C14. But this waveform looks as if the load is too high. What exactly is connected on the RS-232 bus?

  • 0
    Prodigy 10 points

    I made the picture with a 2m cable to a USB-RS232 adapter but the signal is the same when I disconnect the serial cable. It's a 4 Layer pcb. Under the circuit is a GND polygon, 3V3 polygon and again a GND polygon on the bottom layer

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thank you for the right input to solve the problem.  I have duplicate net names on a schematic part that should not be assembled, therefore the DOUT pin is mistakenly connected to a second unused MAX3221 DOUT pin.