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TRS3232E-Q1: TX output waveform is distorted when plugged into PC serial port

Part Number: TRS3232E-Q1
Other Parts Discussed in Thread: TRS3232E

I am using the TRS3232E to interface an embedded system to a serial port on a PC. The TX waveform from the DOUT pin of the TRS3232E looks correct when there is nothing connected (open circuit). When it is connected to the PC serial port, the waveform becomes severely distorted.


It would appear the distortion is related to the loading effect of the PC serial port.

I am using capacitor values recommended in the data sheet. Can the distortion be corrected by selecting different capacitor values for the TRS3232E?

The exact capacitors I am using are:



Thanks for looking.

  • Hi Christopher,

    I've assigned our RS232 specialist to this thread. I believe they should get back to you tomorrow on this.


  • The capacitors look good to me. 3 things you may wan to check: 1) current sourcing capability of the 5V supply (Figure 1 of this blog); 2) the PC's capacitive load (should be less than 2500pF); 3) grounding difference of the PC (shorting TRS3232E ground to the PC's).

  • Hao, thank you for your reply.

    After further debugging, it appears the problem was the PC USB->RS-232 serial adapter I was using for testing. This serial port adapter is either faulty or poor quality and was causing excessive loading resulting in the distorted waveform. I changed to a better USB->RS-232 adapter for the PC and now there is no distortion and the waveform swings full range.

    I also tried the faulty adapter with a different known good board that has a different RS-232 transceiver IC and got a similarly distorted TX waveform, so the problem was definitely the serial port adapter and not the TRS3232E or capacitor values.

    I think the lesson here is to always use a good quality USB->RS-232 adapter!

    Thank you for your advice!