FT232R (clone): RX works, TX only works in parallel to PL2303 chip - SOLVED - Page 1
Forum section: electronics / beginners

author: SmokingResistor
author_rank: Newbie
timestamp: August 01, 2020, 09:00:00 am
content: Hello all,To start: apologies for the formatting. It's my first post here, and I don't fully master how the text is flowing around the pictures...I observe something strange with a TTL converter I bought on aliexpress (the red one in the photo). Given the price, I assume the FT232 chip is a clone.When I hook up the TTL converter to an arduino board to test the TX/RX, the RX works, but the TX does not work. (see the first fritzing scheme)If I then hook up in parallel another TTL converter with a PL2303, both the TX and RX of the FT232 work. (see the second fritzing scheme: TX of FT232 connected to TX of PL2303 and RX of arduino)The PL2303 needs to be powered for it to work. If I remove the VCC connection of the PL2303, the TX of the FT232 stops working again.Any thoughts what may be going on? It wouldn't be a huge financial loss to throw the piece away. But I'm curious to understand what is going on here ...Many thanks for your ideas.

author: Dabbot
author_rank: Regular Contributor
timestamp: August 01, 2020, 09:11:40 am
content: You're connecting the TX of both modules together. These are both outputs. Do not do this.

author: SmokingResistor
author_rank: Newbie
timestamp: August 01, 2020, 09:19:42 am
content: @Dabbot:Thanks for your reply.I know I shouldn't be doing this (connecting the TX of both modules). In fact, it was just a coincidence (mistake) while I was trying to figure out what was going on. But is shows that the TX of the FT232 is not completely "dead". What I don't understand is why the TX of that module is only working when also the other module is hooked up in this strange configuration...

author: Dabbot
author_rank: Regular Contributor
timestamp: August 01, 2020, 09:26:10 am
content: Speculatively, the output on the clone may be half toast, and is only able to source, instead of source + sink.Remove the PL2303 module and place a 1K or 10K resistor between VCC and the TX pin of the clone. See if that works.

author: Ian.M
author_rank: Super Contributor
timestamp: August 01, 2020, 09:40:31 am
content: Dabbot just pipped me to the post!Asynchronous logic level serial signals idle high. That suggests your 'FT232R' isn't driving its TX pin high, but is driving it low when required. You may have damaged it by a previous mis-connection.Try bodging it with a 1K pullup to target Vcc. A 10K pullup may or may not work - it depends on whether the high side output MOSFET's gone open or the low side one's gone leaky enough the high side one can't drag the output to a valid logic '1' level. Scoping the signal or even measuring voltages when idle and when sending a BREAK condition will tell all!N.B. Even if you get it working, I wouldn't trust it at high data rates or for any other application than async. serial.

author: Syntax Error
author_rank: Frequent Contributor
timestamp: August 01, 2020, 09:43:12 am
content: Just as a check, what voltage have you selected on the red module - 5v or 3v3? This is the output voltage (tx) of the red module.There is something odd about those red FT232R modules as they seem to 'back power' when not connected to USB 5V, by taking about 1V8 off of the RX line which might be raised to 2V5. The LED might also be on without the USB connected!Attached is a schematic of how the red module is likely wired.

author: SmokingResistor
author_rank: Newbie
timestamp: August 01, 2020, 12:39:47 pm
content: @Dabbot@Ian.MMany thanks, I think you are spot on:- As a start, I measured between TX and ground of both modules when idle. For the PL2303, that was 5V, for the FT232R it was 0.6V.- So I understand the "mechanism" now: the PL2303 was helping out when wired in parallel by pulling the TX line high. With the PL2303 in place, TX was at 2.3V when idle.- When I removed the PL2303 and put a resistor between VCC and TX of teh FT232R, I got the following results:- w/o resistor: TXvoltage = 0.6V, TX does not work- 10K: TXvoltage = 1.4V, TX does not work- 1K: TXvoltage = 1.7V, TX does not work- 680: TXvoltage = 2.0V, TX does not work- 470: TXvoltage = 2.3V, TX does not work- 220: TXvoltage = 3.0V, TX DOES work(but that's quite low for a pullup I'd say)@Syntax ErrorMany thanks for your answer as well. 5V was selected. (I've updated now the picture in my question, I see that I posted twice the back side of the modules, and not the front side)Just for "fun" I now selected 3V3, but that didn't help to get it working neither.I'll still need to buy a new FT232R, but at least I now understand what was going on.

author: SmokingResistor
author_rank: Newbie
timestamp: August 01, 2020, 12:42:31 pm
content: picture of the front side of the modules

author: Dabbot
author_rank: Regular Contributor
timestamp: August 01, 2020, 12:56:11 pm
content: Quote from: SmokingResistor on August 01, 2020, 12:39:47 pmThat's quite the pullupQuote from: SmokingResistor on August 01, 2020, 12:39:47 pmTry to avoid purchasing dodgy FT232 modules. The fake chips have caused enough fuss already and this behavior shouldn't be rewarded. Why not go for a CH340 or a CP210x, or another PL2303? These are way cheaper and have WHQL drivers available.If you insist on something from FTDI, may I suggest the LC231X module? It's relatively cheap, uses a genuine FT231X chip and has a full breakout:https://www.digikey.com.au/products/en/development-boards-kits-programmers/evaluation-and-demonstration-boards-and-kits/787?k=LC231

author: SmokingResistor
author_rank: Newbie
timestamp: August 02, 2020, 11:23:27 am
content: Thank you for this advice. I just bought some CH340 modules...