Rob,
The DMR-CLEAR_MEMORY_DFU.dfu writes "FF" to each memory location and is only used to clear memory prior to loading a new firmware. Larry has been kind enough to put all the current and past firmware at :
/g/nanovna-users/wiki/home under the "Firmwares" section.
Links to the flash utility and instructions for it's use, as well as various other user guides, can be found here:
/g/nanovna-users/wiki/User-Guides .
The Wiki and Files sub section of this group are a great resource and will answer a lot of questions for any new user of the NanoVNA.
Herb
