Make your Arduino USB for only 1 dollar!

[mpeuser]

Connecting 3V and 5V devices is not such a secret. It is not an issue of voltage but of current. You have to avoid the flow of much more than - say - 100 uA through the protection diodes in the input circuits. To my experience even 1mA will generally not destroy them.

For the voltage of around 1V between 5V and 3V3V + 0V6 a serial resistor of 10k would be more than sufficient protection; I think 1k will also do the job...

[ichilton]

Hi,

Is it good practise to put a 10k inline on both the RX and TX?

Thanks,

Ian

[mpeuser]

This has nothing to to do with good practice but with voltage and current :-) The line transmitting @ 3V3 to a 5V device does not need any resistor, but it will do no harm.

[doshdance]

that depends... but i doubt the arduino will send more than 5 volts to the RX of the PL2303 as what i tested with my multimeter.. they only sent about 4.73v or something...

just test the pins of the arduino.. if u get more than 5v... then u might need to drop it with a ladder. but as far as i know... i  connected it straight from the arduino's TX.
hope this helps...

[mpeuser]

Its not a matter of voltage but of current.. nevertheless do what you want  

[ichilton]

Hi,

I have bought 2 cables, a CA-42 and a DKU-5.

Both arrived and have the PL-2303 chip and when stripped down, both look exactly like this board (and one has the exact same markings):
http://kwartzlab.ca/blog/dw/2010-04-23/reverse-engineering-cheap-ardu

However, for some reason my RXD pin is Pin 4 instead of the 9 in the article which is strange!

Once i'd sorted that though, I can get 5v between GND (6) and VCC (2) and I can connect RXD (4) to TXD (12) and successfully echo characters across in Hyperterm and Realterm.

However, I want to also use DTR so Arduino will auto-reset on program but i'm having trouble finding the DTR pin. I have set DTR and RTS using the toggles in Realterm but going through all the pins with a multimeter between GND (6) and the other pins, none seems to give anything.

Any ideas how I can find DTR (and RTS) and why i'm not seeing it?

The other thing I dont quite understand is why on the above link he uses pin 10, which he had identified as RTS and uses it & calls it DTR in his final pin-out....any ideas why he did that?

Thanks,

Ian

[anachrocomputer]

Thanks for posting that detailed tear-down of the USB/serial cable!  I have two like that one, and the information will be very useful.  So far, I've cut the end off one cable, but the other is intact -- not for long!

[borref]

Any ideas how I can find DTR (and RTS) and why i'm not seeing it?

The cables I've converted don't look quite like yours.

The trick to finding DTR/RTS however is to look at the datasheet for the specific Prolific chip used and then check to see if the DTR pin is brought out to some nearby pad. On mine it was not, so the only option left was to solder a tiny wire from the chip pin itself.

[ichilton]

Hi,

The trick to finding DTR/RTS however is to look at the datasheet for the specific Prolific chip used and then check to see if the DTR pin is brought out to some nearby pad. On mine it was not, so the only option left was to solder a tiny wire from the chip pin itself.

The problem is you can't see the chip as it's covered with one of those black blobs - so all you have is the pads on the pcb

Ian