sumguy:
It is necessary because the OP's test code has no protocol to eliminate erroneous data, that's basically what Robin2 was trying to explain. You say you don't do anything ieee488 but I am guessing you have a protocol of some kind. Setting DTR true is not a bad option at times anyway, it clears the serial buffers and syncs the PC and arduino to a clean start.
Thank you for the explanation, but I must admit I still don't understand why setting DTR to true was necessary for the OP.
The PC setting DTR to true before the COM port is opened causes the Arduino not to send 0x0f ; otherwise, it does ?
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