Serial Port communication to PC troubleshooting

Electrical Engineering Asked on December 26, 2020


The goal is to bridge a serial port measurement device into a USB PL2303 device. Various degrees of success were observed and I hope the community can point me in the right direction in terms of troubleshooting, based on the data / observations below.


A serial port measurement device (9600-8-N-1) has two outputs: each output sends a trio of bytes every second through:

  • green wire: TTL
  • blue wire: RS-232

The first byte indicates the qualify of the data and a good measurement is indicate with 0x80.

PL2303 HXD Description

I have successfully connected the TTL wire to PL2303 devices and have run into unusual results wiring the blue RS-232 wire to a pink PL2303 and believe it to be a HXD device:

  • Device Blue Wire:RS-232 <==> White Wire (PL2303 HXD)

HXD Description:

  • Supports RS-232 serial interface with Programmable baud rate from 75
    bps to 12 Mbps

  • Supports RS422/RS485 interface

  • Extensive flow control mechanism like adjustable high/low watermark
    level, automatic hardware or software flow control, and inbound data
    buffer overflow detection

  • This product is designed for laboratory, product testing, low-cost
    MCU communications and other applications, there are four lead,

HXD Wiring:

  • Red +5 V

  • Black GND

  • White RXD

  • Green TXD

  • Yellow RTS

  • Blue CTS


The first 3 lines show RS-232 results. 0x80 (valid data) starts at the end of the 3rd line:

enter image description here

Said measurement serial-device has only 2 additional wires (power / ground) for a total of 4 wires.

Assuming that there is nothing wrong with measurement device, (it has been tested good with other RS-232 adapters). I inherited PL2303 HXD and although I can lookup a data sheet, I do not have the original data sheet and can not be certain I have the correct data sheet.


Does the PL2303 HXD need to be configured somehow to 9600-8-n-1 or does it somehow default to the configuration or detect the communication protocol? Any diagnostic questions or insight that would explain why the HXD does return good data are appreciated. Thank you

Perhaps the HXD model is not correct for RS-232 and the RS-232 design requirement should select a PL2303-TA? Any clarification or insight that enables choosing the TA over the HXD (or vice versa) is appreciated.

One Answer

Based on the in-line post header style connector on the pink PL2302 cable, I suspect that the cable uses TTL levels, not RS-232. The TTL signal levels are inverted relative to RS-232.

I would expect a cable with RS-232 signal levels would use a DE-9 connector.

Answered by Peter Bennett on December 26, 2020

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