USB to Serial Adapter (PL2303 XA/HXA) Outputs 5V on TX/RX lines - How to get 3V3 - Page 1
Forum section: electronics / beginners

author: donna9
author_rank: Newbie
timestamp: September 21, 2017, 06:55:28 pm
content: Hi guys. Totally n00b so go easy.I bought this USB-to-serial adapter (see atachments). It uses the Prolific PL2303 XA/HXA chipset. I am attempting to use it to communicate with my bricked routers serial port. The problem is i noticed the TXD and RXD lines output at 5V level. My router only wants the TXD, RXD, and GND pins to be connected..the router has a 3V3 pin line too but i am told not to connect to that pin from the USB-serial adapter. When i use my mulitmeter to measure voltage level i see my router's TXD and RXD lines read at 3V3 volts. So i want my USB-serial adapter's TXD and RXD pins to also be outputing at 3V3 volts otherwise the fear is the serial connection wont work or worse, it damages my already bricked router.Does anyone know how to mod this adapter with just a solder gun so that the TXD and RXD lines output at 3V3 levels instead of its current 5V?I would hate to have to use some voltage regulator module in between the TX to router and RX to router pins (if that is how it works) just to drop the 5V to 3.3 because where i am located it would require me ordering one which could take a few weeks or more. Ideally, i would like to get this solved with what i have with me which is simple soldering tools.If you need more pictures/closer view please ask. I will update this post within a few minutes.

author: cdev
author_rank: Super Contributor
timestamp: September 21, 2017, 07:52:47 pm
content: "usb-stc-isp" adapters vary but the difference in voltages isnt so large.Google brings up a great many articles on level shifting between 5 and 3.3 volt devices using various meansDon't connect the power line..This discussion at sparkfun brings up the point that the two logic levels are close to being compatible, but not quite always.https://learn.sparkfun.com/tutorials/logic-levelsanother discussion is herehttp://jamesreubenknowles.com/level-shifting-stragety-experments-1741Here is anotherhttps://hackaday.com/2016/12/05/taking-it-to-another-level-making-3-3v-and-5v-logic-communicate-with-level-shifters/There is a very simple circuit used to connect 5v ATmega devices to RPIs (3.3 v), for example in the discussion below..under "Simple diode circuit" (EDIT: thinking about the application, a flash that needs to be 100% error free or router is bricked until you make or buy a JTAG cable, its perhaps not a good circuit unless you are sure that the output of the level shifter's levels are within spec for your device.) Your device may even work fine with 3.3 volt logic unmodified. My CP-2102 USB-UART has on many occasions when I forgot to change the jumper.http://elinux.org/RPi_GPIO_Interface_CircuitsHere is an application note about interfacing the two different logic families in the i2c contexthttps://www.nxp.com/docs/en/application-note/AN10441.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: September 21, 2017, 09:26:18 pm
content: See datasheet:http://www.usconverters.com/downloads/datasheets/pl2303.pdfLift pin 4 (serial port Vdd) and strap it to pin 17 (3.3V regulator output)

author: donna9
author_rank: Newbie
timestamp: September 21, 2017, 11:11:48 pm
content: Wow much appreciated @cdev and @Ian ... very helpful!