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I bought an uart ttl to usb 2.0 adapter on ebay which I tried out yesterday with no luck. I measured all the pins and found to my surprise that the RX pin actually carries 3.3V. This can't be right as the unit I intend to communicate with has 5V on its TX and 0V on its RX. Can my adapter be dysfunctional?

Secondly, I wonder if it doesn't matter that one seem to carry 3.3V on its TX and the other 5V, can the respective devices handle this in any cases or must it be carefully matched?

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  • \$\begingroup\$ (1) Ask the eBay seller. EE.SE is not his volunteer support desk. (2) That eBay auction shows a schematic. Was anyone able to figure out what that schematic has to do with a USB-to-UART adapter? \$\endgroup\$ Apr 15, 2015 at 6:23

2 Answers 2

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Because the minimum output level for a logic 1 using 3.3V level logic is 2.4v; and the minimum logic 1 level for 5v logic is 2.0v.

As far as your UARTs are concerned, I would test out each one individually in a loop-around configuration; that is tie TX to RX of the same adapter, and output a character and see if you can read it back. Then repeat for the other adapter. They might tell you where the problem is located.

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  • \$\begingroup\$ You're right. Loop back test works. If I use an external ground RX is not giving voltage, but if I use the ground pin on the adapter to RX I see 3.33V. I guess there's a problem with my methodology that I don't understand \$\endgroup\$
    – 3molo
    Apr 15, 2015 at 11:31
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According to the datasheet of the chip used on the board you are using the chip runs on a 3.3v supply, meaning it would send out 3.3v on a logic high. The data sheet also said that the I/O could go up to 5.8v, so it's fine that you are communicating with a 5v device.

One potential issue you may be having is that the receive from the USB module needs to go to the transmit of the device you're communicating with. So RX -> TX and TX -> RX

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  • \$\begingroup\$ Im positive I connect RX to TX and vice versa. And both TX and RX definitely has 3.3V on the adapter, while the unit I want to connect to only has voltage on TX. Are you saying this is fine? Ie RX on the adapter with 3.3V can connect to TX on the unit which has 5V? That is connecting two positives, I just don't understand how that can work \$\endgroup\$
    – 3molo
    Apr 15, 2015 at 5:59
  • \$\begingroup\$ I have seen some devices marked such that the markings are "Connect TX to here" ie it is the RX pin. The idea I presume is it's easy to connect TX to TX and RX to RX making it easy for the beginner. If the pins you are connecting together are both sources (TX) then you know it's not going to work. This is why a good datasheet is so important. \$\endgroup\$
    – Spoon
    Apr 15, 2015 at 7:38
  • \$\begingroup\$ On the adapter, both TX and RX has 3.3V.. On the unit I want to communicate with, TX has 5V while RX has 0V. \$\endgroup\$
    – 3molo
    Apr 15, 2015 at 9:03
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    \$\begingroup\$ on the adapter try a loop back test (connect TX to RX) and then in a terminal program you should see your characters come back to you (disconnect the loop back afterwards just to make sure Local Echo mode is off) \$\endgroup\$
    – Spoon
    Apr 15, 2015 at 11:35

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