How can I write/read EEPROM on EMX?
There’s no Eeprom on EMX module. There’s flash and battery RAM.
That is very bad. 
we had wrongly downgrade EMX to Embedded Master firmware, now the USB port is broken, fortunately we can use serial port to upgrade the devices to EMX firmware. But and all devices lose serial number information. I thought the serial number is stored in EEPROM.
Thanks. Yes, I use this API to get serial number, but after downgrading the firmware, we only could get empty string with this API.
Maybe you can know that you are using EMX with: http://www.ghielectronics.com/downloads/man/Library_Documentation_v4.2/Premium/html/72facb79-86c3-2c59-013f-25c8bb0cae32.htm
Then why not use mac adress as serial number?
@ leforban -
Thank you. I checked reference manual that GetSerialNumber is obsoleted for EMX. Now we have to use MF 4.1, because our software runs on embedded master and EMX devices. Embedded master only support up to 4.1.
The reason that we don’t want to use mac address as serial number is, after we upgrade tinybooter&GHI firmware, Mac address on all devices (both on EM & EMX) become a same one: 00-1A-F1-00-42-0D. Because a lot of devices might be upgrade to a new GHI firmware, we use serial number to generate a unique mac address for Embedded Master devices. Now seems we have to update the Mac address manually with MFDeploy.
Another sad thing is that GHI guys don’t implement Microsoft.SPOT.Hardware.SysetmID.SKU. so we could not utilize this number too.
May be you can check for the number of IOs (if there’ different) or use a GPIO with a jumper to deifferentiate your product