Long times ago I bought on the ebay a cheap rs232->usb converter, today I wanted to use but I realized that I don't have the pinout. This is the converter how can I figure it out the pinouts, does anybody has experience with this?
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Since USB does not have negative voltages, I think it's probably unsafe to assume that pin one is ground.
I would guess that the pin you say has -3.3V is actually ground. You can confirm this by measuring the resistance between the pins and the USB ground pin (you may need to cut up a USB cable. Alternatively, the USB shield may be grounded, so try just probing with one lead on the USB connector metal casing).
Going from there, I would write a little script that outputs a continuous serial stream, and poke around with an oscilloscope until I found the TX line.
Then, I would take a device (an arduino, for example), and set the arduino up to output a continuous stream of serial bytes. Then, with a 10K resistor in series with the arduino TX, you poke around on the pins until you see that you are receiving data from the USB-serial interface. That will tell you where the RX line is.
The alternative option is simply to trace out the PCB traces. The CP2102 datasheet is freely available, and I doubt that board is more then two layers. It shouldn't be too much work to follow the traces back from the connector.
You may need to probe some of the IC pins to verify the path of any traces that run under the IC, use a needle in a clip-lead for that (probing the pins on a QFN is fiddly - the pins are tiny).
There are literally hundreds of these gadgets out there. You'll really need to check pin by pin, tracing to the IC.
I have several (with a different PCB, unfortunately), and the PCB silkscreening is misleading on mine to boot (Tx and Rx are from the perspective of the UART, not the IC - Tx is the incoming connection, Rx the outgoing one ... sigh ... that took a while to sort out)
That being said, many CP2102-based interfaces share these six common signals on the UART side: GND, 3.3V, 5V, RXD, TXD and reset. These should map to the pads opposite the USB connector.
The pins around the top and bottom are for ancillary functions like RTS, CTS, DCD, DCR, etc. and are more commonly used when the IC is paired with an RS232 driver.