I am pretty sure that there is a kind of boot-strap OS in the S7-300 CPUs. And the boot-strap OS will detect a Siemens MMC card and if there is a firmware update. If found it loads the firmware update into its internal flash memory.
You always need a Siemens MMC card in an S7-300 CPU, even a small 64 kB will do. For a firmware update it has to be the 4MB size which is expensive.
You always need a Siemens MMC card in an S7-300 CPU, even a small 64 kB will do. For a firmware update it has to be the 4MB size which is expensive.