I have a cheap USB to 3.3V TTL FTDI Cable that I bought on Ebay. It has been giving me some troubles ever since I got it, burned couple RFM12B modules along the way. I decided to hook it to my scope to see what is going on:
So the thing provides 5V on the power line, whilst the TTL levels are 3.3V. Yuck, why would they provide 5V on the power line??!! I have another one that is better quality and it provides both 3.3V TTL and 3.3V on the power line. The 5V effectively fried the RFM12B on the RFM2Pi board that I was trying to program within seconds..
Just a warning to you, measure before use 🙂
[EDIT]: Checking the cable datasheet reveals that these are meant to supply 5V on the VCC. So it was my mistake all along for not reading the datasheet, I have assumed all along that a 3.3V TTL cable would also supply 3.3V on the VCC, furthermore I had previously read here, that this is an unwanted mix-up. As I mention I do have another one, that has a jumper to chose between 3.3 and 5V TTL, that one does indeed also supply 3.3V and 5V respectively on the VCC. Again, a word of warning if you deal with 3.3V sensitive equipment and these cables.
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