https://devzone.nordicsemi.com/f/nordic-q-a/14153/how-to-update-dfu-from-sdk-6-1-using-nrf-toolboxThere are a couple of threads in the Developer Zone which mention backwards compatibility issues for nRF51822 BLE with DFU. Some of these are focussed around MCP (Master Control Panel), and suggest using nRF Toolbox (our preference, anyway), and anotheren-USTelligent Community 13Tue, 31 May 2016 13:00:28 GMThttps://devzone.nordicsemi.com/thread/54080?ContentTypeID=1Tue, 31 May 2016 13:00:28 GMT137ad170-7792-4731-bb38-c0d22fbe4515:9ac8a8b7-1ab1-463b-b9f2-bcdabb0f79a8Hung Bui<p>Hi Clayton,</p> <p>Have you tried what suggested by Vidar to add BANK_UNUSED = 0xFFFFFFF, in the bootloader setting structure. Note that the init packet is not obligated, so you can actually send bootloader+softdevice without init packet. Please have a look <a href="https://devzone.nordicsemi.com/blogs/696/compatibility-between-dfu-bootloader-and-dfu-maste/">here</a>.</p> <p>Please get back if you have any trouble.</p><div style="clear:both;"></div>