Yamaha DX-21 – Firmware OS Update Version 1.5
This is firmware update 1.5 and is an official released operation system by Yamaha. This operation system update/upgrade requires one eprom chips.
How to check current version:
Hold voice button  and  on side A, while powering the device and release both buttons, when text is displayed.
There is no information about firmware revisions for DX-21. There could be devices with OS 1.5 or lower, but sadly no changelogs have been published therefore no documentation about the changes from 1.0 to 1.5