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Author Topic: [solved] USB to Serial TTL Question  (Read 1533 times)
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This is just killing me. I have several USB to serial TTL devices.  SOme put out only +3.3 volts, others only +5 volts while another has a jumper that will allow you to Vcc out either 3.3 or 5 volts.

My read dilemma is this on PL 2303 I got off ebay.   It has a Vcc +5 and +3.3 out put the Tx/Rx always measure +5 volts not matter which Vcc out I will use.

http://www.ebay.com/itm/251082091781?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

Would this work on the Arduino products?  I believe the want +5 volts Vcc in if I did a breadboard but will the +5 volts Rx/Tx hurt the Microprocessor or is it 3.3-5 volts tolerant?  The data lines.

Thanks

« Last Edit: October 05, 2012, 09:10:03 pm by pcfr33k » Logged

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The data lines are good up to the voltage level of the Vcc line.
So if power from 3.3V, your data out lines will be ~3.3V and any incoming lines should be <=3.3V.
If you power from 5V, same thing: outputsp will be ~5V, and any coming lines should be <=5V.
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The data lines are good up to the voltage level of the Vcc line.
So if power from 3.3V, your data out lines will be ~3.3V and any incoming lines should be <=3.3V.
If you power from 5V, same thing: outputsp will be ~5V, and any coming lines should be <=5V.

Thanks for replying.   Well that was my point on the one USB to Serial device I have.  You cannot jumper it to put out 3.3 or 5 volts but it has a Vcc reading 3 volts and another Vcc that reads 5 volts.  The Rx/Tx read 5 volts either way.

So use the 5 volt Vcc line then?

What would happen if I used the Vcc 3 volts and the data lins Rx/Tx are putting out +5?
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The PL2303 is a really lousy chip, I wouldn't waste any time on it.

it has a Vcc reading 3 volts and another Vcc that reads 5 volts. 
Without a schematic, it is going to be tough to solve this strange puzzle.  Since it doesn't look like there is a regulator onboard, it could be that the 3V3 isn't VCC, but a regulated output from the PL2303.
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"What would happen if I used the Vcc 3 volts and the data lins Rx/Tx are putting out +5?"
Tx won't put out 5V, it would only put 3V.
If the other end drove Rx to 5V, then the Rx pin would get damaged - the uC only tolerates Vcc + 0.5V. 5V is well above 3.5V.
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Thanks again for the replies.  I had a link to the product I bought in my first post.  I measured each line. Vcc +5 measures +5.  Vcc 3.3 measures 3.288   Rx and Tx measure just under 5 volts each.  I used the ground as reference when measuring each pin.

Something sounds wrong with this device.

I have another USB to serial that has a jumper one for +5 and other for +3.3 so they measure correctly.  When +5 is enabled I measure Rx/Tx juts under 5 volts and when I enable +3.3 I measure just under 3.3 volts for the Rx/Tx using ground for reference.
« Last Edit: October 05, 2012, 08:49:11 pm by pcfr33k » Logged

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What sounds wrong? Levels sound good. Hook up +5, Gnd, connect Tx to Arduino Rx and Rx to Arduino Tx.
Press reset on your board when the IDE says "Compiled xxx of 32xxx bytes".
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So if I use the unit that has +3.3 and +5 Vcc out and use the +5 Vcc out and the Rx/Tx that measure under +5volts I should be ok?

What the heck is the purpose of having +3.3 Vcc if Rx/Tx does not change to 3.3 and remains under 5?

I am trying to figure out why this USB to Serial was made this way and how it could be used practically!!!
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"So if I use the unit that has +3.3 and +5 Vcc out and use the +5 Vcc out and the Rx/Tx that measure under +5volts I should be ok?"
Yes.

"What the heck is the purpose of having +3.3 Vcc if Rx/Tx does not change to 3.3 and remains under 5?"
Some devices need 3.3 for Vcc but have 5V tolerant IO pins.

"I am trying to figure out why this USB to Serial was made this way and how it could be used practically!!!"
Ignore the 3.3V.
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Thanks!!

How do I modify the original subject to show [solved]
« Last Edit: October 05, 2012, 09:07:43 pm by pcfr33k » Logged

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You got it figured out it seems?
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