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This question already has an answer here:

I have a USB-to-RS232 converter. The GND, TX and Rx pins of the RS232 I connect to GND, Rx and Tx on a development board I have. I've set the right baud rate, flow control and other settings in my HyperTerminal. However, all I receive is garbage data. I looked it up on the Internet and people who used a USB-to-UART converter, like the one here http://cpc.farnell.com/ftdi/ttl-232r-rpi/cable-debug-ttl-232-usb-rpi/dp/SC12825, have had no problems with it. I obviously I am missing something, any help in this direction would be really appreciated :)

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marked as duplicate by Cornelius, tcrosley, Daniel Grillo, Wouter van Ooijen, JYelton Jul 28 '14 at 20:51

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    \$\begingroup\$ Agreed this is duplicate, plus the linked answer above has a great answer. \$\endgroup\$ – Funkyguy Jul 28 '14 at 19:23
  • \$\begingroup\$ Also be sure that if you are using asynchronous mode with your controller that you use an external oscillator. If you are using an internal RC oscillator then you will likely run into problems since the tolerances are typically high. If this comment does not relate to your application then forgive me. \$\endgroup\$ – sherrellbc Jul 28 '14 at 20:12
  • \$\begingroup\$ @sherrellbc Many modern micro-controllers (all small Cortex chips that I know) have a < 1% tolerance on the internal oscillator, which is sufficient for reliable asynch communication. \$\endgroup\$ – Wouter van Ooijen Jul 28 '14 at 20:35
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USB to RS232 means RS232 levels (+-9Vdc). USB to UART usually means a logic family levels (0 to 5V, 0 to 3.3V etc)

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    \$\begingroup\$ Additionally, the sense of the signals is usually opposite between "logic level" and RS232 implementations - the level translator chips customarily invert. \$\endgroup\$ – Chris Stratton Jul 28 '14 at 20:03
  • \$\begingroup\$ To expand on this if you have connected a RS232 device to a logic-level serial interface you have very likely fried it. \$\endgroup\$ – Peter Green Oct 13 '16 at 0:39

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