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Didn't we cover this last year ?
I found then that the USB -> SERIAL port adapters (probably for Macs) don't work with my WinXP laptop, but the COM port adapter does, albeit not as consistently as a good hardware COM port. Possibly, the SERIAL adapter didn't return enough (any ?) non-serial status lines, while the COM port adapter *does* simulate their return in USB, with a latency. But, there is an issue about how long it takes the adapter and the USB port to pass information back to the laptop, as mentioned in my USB->COM adapter's manual.
The algorithm for programming a single byte in a PIC calls for the programmer (hardware, perhaps the software in our case) to repeat a programming cycle of several tries, then read the just programmed byte, and then reprogram it until the byte is programmed correctly.
If the programming device can't get the feedback it needs, in time, it might just reprogram until (by chance ?) it sees the correct byte. This could be an accident of the PC's timer interrupt arriving as the PIC sends, slowing the programming logic, allowing the feedback to be perceived. In each of the bytes to be programmed, this could take forever (for instance 10 -12 minutes ?). This appears to be the case in the thread above. We'd be well advised to take a laptop with a known good hardware COM port to any competition, to avoid missing a match while reprogramming.
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