Bootloaders seem very time sensitive to the variable delays when using USB to RS232 converters. Colleagues and I have tried at least half a dozen different looking adapters with netbooks, laptops notebooks and desk PCs. NONE gave 100% reliable results. Some mostly work, some never work. They are all Chinese and are generally fine for dumb ASCII communications but whenever accurate timing between sending a character and getting a reply is needed then it's back to an old PC with dedicated RS-232 DB9 connectors.

It was my design practice to always fit an RS-232/TTL port to every design and use the MCSP bootloader for field bugfixes. I have abandoned that now and use the MeLabs U2 USB programmer for all code programming and updates. I have requested Mecanique to have a look at relaxing the timing on the MCSP bootloader but that message sank without trace.

HTH
Brian