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.


1 Answer 1


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.

  • \$\begingroup\$ Well, when I connect long distances over 120 feet, I do use DB9 connector and 9 pin double shield cables. I do use MAX232 chip to convert 3V3/5V0 to shfit to +-12 volts signals. But that is overkill unless the other side insists on +1-12V. Perhaps the OP might like give us the links to your devices to clarify. \$\endgroup\$
    – tlfong01
    Aug 11, 2020 at 6:42
  • \$\begingroup\$ @PeterBennett I believe that a DB9 connector indicates a RS-232 protocol, I would think that it is possible to use another physical connector for RS-232 communications. I updated the OP with the electrical specs of the pink PL2303 (HXD). Does it make sense to try PL2303TA found on eBay? \$\endgroup\$
    – gatorback
    Aug 11, 2020 at 15:17
  • \$\begingroup\$ @gatorback: It is certainly possible to use a connector other than DE-9 for RS-232 connections, but DE-9 is by far the most common. (and it is DE-0, not DB - the DB connector is much larger, and holds 25 pins. The DB-25 connector was the standard RS-232 connector when I started in electronics in a previous century) \$\endgroup\$ Aug 11, 2020 at 15:24
  • \$\begingroup\$ @gatorback - please really read this answer, you keep linking to what is clearly wrong sort of product with individual leads intended for Arduino/Pi/MCU projects, if you want RS232 buy something with a 9 pin D-shape connector then cut it off or better make an adapter cable. And shopping questions are off topic anyway. \$\endgroup\$ Aug 11, 2020 at 15:45
  • \$\begingroup\$ @PeterBennett. Thanks for the response / clarification. I am trying to bridge an RJ11 terminated RS-232 device into a USB to serial cable. I plan to terminate the USB to RS-232 Serial cable with RJ11 and bridge the two with a coupler. Because 20+ units are to be built, choosing a design (PL2303 cables) will drive cost and labor outcomes \$\endgroup\$
    – gatorback
    Aug 11, 2020 at 16:09

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