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 $3 as well, but just made a break-out board for the max232 to Arduino.. only reason I bought the USB cable to RS232 was so that I can make custom arduino's without needing to solder the surface mount USB-to-TTL chip!
The MAX232 chip is amazingly easy to hook up.. I had no experience with them... my first time, just followed the schematics, plugged the RS-232 cable in, hit upload... hit reset on the board, and bada-bing bada-boom, blinking lights! 
I'm confused...
Protocols are the same except for logic levels, 5/0 vs -12/12 for high/low respectively. So what we bought - less the Max chip is what we desire.
There are of course plenty of boards/cables/dongles available that will do direct USB/TTL appropriate for MCU's of all flavors, but I'm not aware of any at this price point (2$).
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 .
Tanks
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.
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:
http://www.dealextreme.com/details.dx/sku.24512
Also I found a datasheet for the Prolific chip (Pl-2303) here:
http://user.chollian.net/~mines/pl2303.pdf
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:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1237497869
https://web.archive.org/web/20210308144706/https://www.uchobby.com/index.php/2009/10/04/diy-usb-to-serial-cable-for-3/
(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.)
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:
http://profmason.com/?p=539
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]