USB to RS232 converter - Page 1
Forum section: electronics / general

author: Wawavoun
author_rank: Regular Contributor
timestamp: November 27, 2022, 07:20:37 pm
content: Hello there,I try to use a usb to rs232 cable converter. It come from Amazon and had a '340' mark molded on the db9 side.This the driver Linux load when it is connected (ch341).Loopback (rx connected to tx) is working well but no way to communicate with my board.The scope show a idle level of 0 V on the tx pin and during transmit pulses goes high to +5V.As far as I know idle level on RS232 is between -5 to -12 V and pulses goes to +5 to +12 V during transmit...Looking at the schematics 0 V cannot do anything to the receipt circuit.Is there something to adjust (in the driver) to get the good rs232 levels ? Or may be this cable lack a Max232 or similar for level shifting ?Thanks.Philippe

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 27, 2022, 07:43:38 pm
content: Quote from: Wawavoun on November 27, 2022, 07:20:37 pmWhere are you connecting your oscilloscope ground in this measurement? DB9 connector pin 5?Quote from: Wawavoun on November 27, 2022, 07:20:37 pmIt's possible that someone sold you a USB to 5V TTL UART cable with a DB9 connector...

author: Ian.M
author_rank: Super Contributor
timestamp: November 27, 2022, 08:04:35 pm
content: RS-232C calls for minimum signal output levels of +/-5V, so although 5V is legal for the space (logic 0) level, its mark (logic 1) level of 0V is invalid. Also TTL serial with RS-232 framing almost invariably uses mark high logic, i.e. idles high, and is inverted with respect to RS232 levels, so it almost certainly isn't a TTL level UART adapter.If your converter is still within its returns period, return it to Amazon as defective - either its fake and they've put in a cheap logic inverter rather than a RS-232 line driver/level converter (like a MAX232 but with more lines), or the level converter is faulty and its negative rail charge pump is not running.

author: Wawavoun
author_rank: Regular Contributor
timestamp: November 27, 2022, 08:21:59 pm
content: Yes the scope is connected between pin 3 (signal) and pin 5 (gnd).To Ian : the max232 is a level shifter but also an inverting device. So to get a -5 V to -12 V idle the ttl level should be +5 V. Is that why you say I am not in front of a 'pure' ttl device ?To late for return but certainly you are right : faulty devices.Did you know a rock solid device with also rts, cts, dtr etc... transmission and not only rx and tx ?Thanks for your advices.Philippe

author: Karel
author_rank: Super Contributor
timestamp: November 27, 2022, 08:37:24 pm
content: https://ftdichip.com/products/us232r-100-bulk/https://www.digikey.com/products/en?keywords=768-1074-ND

author: Ian.M
author_rank: Super Contributor
timestamp: November 27, 2022, 09:13:36 pm
content: Quote from: Wawavoun on November 27, 2022, 08:21:59 pmYes. That's exactly why I don't think its a TTL level cable. Of course you could use it as one if you added eight inverters!Sorry, my full 9 pin Nikkai USB COM port adapter that was reliable for most of the last decade turns out to have a fake Prolific PL2303 chip in it so to get it to communicate under Windows 10, I had to install old drivers, which have to be reinstalled every time MS pushes out a major OS update, and manually selected in Device Manager each time its plugged into a new USB port!I am therefore also considering looking for a replacement.

author: Wawavoun
author_rank: Regular Contributor
timestamp: November 28, 2022, 07:29:39 am
content: Thanks for all the explanations !I will change this cable for a better one following the links in preceding message.About your driver problem : consider Linux?Regards.Philippe