3$ USB RS232 Cable for Arduino!

Hi guys ,
I decided to be smart and buy a USB RS232 for 3$ (inc shipment) and make it work with arduino .
The problem was +/-12V from the PC and +5v/0V required by the arduino .
I opened it up and found a similar chip to the MAX232 (after the USB chip) . in any case I removed 4 caps and the voltage levels are just right! 5V /0V to arduino. - Great ! but it didn't work .
After 2 hours of debug I found the problem - The voltage levels are invered! damn it . high is low and low is high . when I short tx and RX I get an echo on the PC but due to double inversion in Tx and then in Rx .
Does any one have any idea how I can invert th driver / RS232 on windows to make it work so we can all have very cheap RS232 USB cables ?

Remove the MAX232 in the USB lead and feed the input straight through to the output.

I decided to be smart and buy a USB RS232 for 3$ (inc shipment) and make it work with arduino

I did the same some time ago, but haven’t been able to use them as I intended - yet.

I figured the TTL side of the Max chip would have the right voltages and logic levels (they come in 3v3 and 5V flavors - mine were 5V). Probing the signal from the pins on a scope (with data flowing) looked promising.

Then I probed around the small circuit board for solder pads or through holes connected to the pins of interest (Tx, Rx, DTR), but came up short (power/gnd was ok). Next idea was to solder tiny wires directly onto the Max IC pins, but I gave up - the legs on these SMD parts are just too small (for my eyes/hands at least).

One possible solution is to build a small RS232 interface board (e.g. a Max232 DIP with 4 capacitors) and connect this in between the Arduino and the converter, but this was not quite what I had in mind.

If you find a better solution - I would love to see it.

I'm confused... you're trying to hook up a RS232 device at 5v, to a TTL compatible device? Those are two totally different protocols I thought? (or is one just based at 12v/-12v while the other is 5v?)

The USB to RS232 cable isn't going to work without a RS232 to TTL converter... like the Max232 like was suggested above.

I bought a USB to RS232 for about $3 as well, but just made a break-out board for the max232 to Arduino.. only reason I bought the USB cable to RS232 was so that I can make custom arduino's without needing to solder the surface mount USB-to-TTL chip!

The MAX232 chip is amazingly easy to hook up.. I had no experience with them... my first time, just followed the schematics, plugged the RS-232 cable in, hit upload... hit reset on the board, and bada-bing bada-boom, blinking lights! :slight_smile:

I’m confused…

Protocols are the same except for logic levels, 5/0 vs -12/12 for high/low respectively. So what we bought - less the Max chip is what we desire.

There are of course plenty of boards/cables/dongles available that will do direct USB/TTL appropriate for MCU’s of all flavors, but I’m not aware of any at this price point (2$).

I will go with Grumpy_Mike.
THey are very small (and all filled with silicon now - I was too sure of myself).. but I will give it a try .

hey, thanks for pointing these out. I'm going to try one for my AVR butterfly.

The pins are too small to connect from in to out .
I am looking for source code for the pl2303 driver - but cant find them .
If I could then we could have a driver for this cool RS232 cabel ...

There's downloads for the drivers for RS-232 cables, I just searched google and downloaded the first one.. worked like a charm.

I'm not so sure they have the Source-code available, but maybe, didn't search for that:)

I am looking for source code for the pl2303 driver - but cant find them .
If I could then we could have a driver for this cool RS232 cabel ...

That would be neat. :slight_smile:

The driver source however may not cover what you need. The issue here is that start/stop bits are added/stripped by the prolific chip itself and so reversing polarity would require reprogramming the chip. I don't know if this can be done and even so it may not be possible to invert logic levels.

Driver source is available for Linux, but I have not seen a Windows version.


The device I've been using is this:


Also I found a datasheet for the Prolific chip (Pl-2303) here:


With this info I was able to get it working for Rx and Tx. RxD (Pin 5 on Pl-2303) was broken out to a nearby solder pad and so fairly easy to connect. TxD is pin 1 on the Pl-2303 and I was able to solder a small wire directly to this pin.

I have not been able to connect DTR (Pl-2303 pin2) however which is needed for auto-reset to work.

I used eaxt same component - cool ha 3$ ... but I just removed 4 caps and got the correct voltage for all pins but - in invert !
too bad . I will do it your way .
What proj you working on ?
I am thining of a robot to clean up the house or a laser cutter fomr a printer ...

If you want to keep it as is (no caps, inverted logic) you may still be able to use it with a modified software serial driver on the Arduino.

You can get a non-inverting, not-12V USB to serial converter for about the same $3 price, only it will be disguised as a "cell phone cable." See:

(not ALL usb->cellphone cables have a USB/serial converter, but this particular CA-42 cable for Nokia certainly does! 3.3V levels, usually. Should be OK with most arduinos.)

The USB BUB is available with or without a USB A to mini cable. This is the same one that your digital camera connects with, so you can use the one you already have or get an extra for your camera. All for less than the price of competing adapters or the FTDI cable.

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I just ordered it frm ebay for 2.66 inc ship .

You're making this harder than it has to be. For RS-232 logical 1 is a negative voltage and 0 is positive. For Serial TTL logical 1 is 5V and 0 is 0V. An easy way to convert RS-232 to Serial TTL is to just use inverters powered with 5V. See this page:

Another way to do it is to buy a Nokia cellphone cable and modify it, no inverters required.

Another way to do it is to buy a Nokia cellphone cable and modify it, no inverters required.

That is literally exactly what's happening.......

[edit]Wow, I completely zoned out. I totally thought I was replying in this thread! My bad ;D![/edit]