When programming the Pro Mini using this USB-to-serial adapter the boot loader only works once. After the first programming the programmer is out of sync or not responding.

To be able to program the Pro Mini again I have to burn the bootloader using my Uno as an ISP.

Programming the Pro Mini through an Uno does not present the same problem.

Are the lock bits not being set properly or somehow overridden?

Usb to serial module

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    Does that adapter really have a DTR signal? – Mikael Patel Dec 22 '15 at 20:20
  • Please show your full wiring. – Majenko Dec 22 '15 at 20:59
  • I actually have the same adapter and it does not have the DTR signal. Therefore it cannot be used. A reset signal cannot be generated. The adapter has 3.3V, RST, TXD, RXD, GND and 5V. The RTS is not the same as DTR. – Mikael Patel Dec 22 '15 at 21:10
  • It does NOT have a DTR signal. Thank you for your help. Will order one that does. – Martin Nilsson Dec 22 '15 at 21:20
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    Make sure you add the inline 100nF capacitor in the DTR line or the newer versions of avrdude will fail miserably. – Majenko Dec 24 '15 at 12:59

The module I had does not have a DTR signal. I mistook the RTS for DTR. Ordering correct module.

  • You have received faulty advice. The RTS signal is usable, and in fact used in official guides. Your problem is likely something else. – Chris Stratton Dec 26 '15 at 5:49

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