Tempo Hold is a new feature of the DD-500, as of the 1.10 System Update. This feature allows you to set a Global Tempo setting, so that whichever patch you recall, the DD-500 will always be at the same Tempo, no matter which preset the unit is currently in.
To turn it on:
1. Press EDIT.
2. Press the Down Arrow to select the SYSTEM Menu.
3. Pres EDIT.
4. Press the Down Arrow to select the TEMPO HOLD parameter.
5. Turn the TIME/VALUE knob to enable the Tempo Hold feature.
If you need the DD-500's Ver. 1.10 System Update, you can get it here:
https://www.boss.info/us/support/by_product/dd-500/updates_drivers/60561dd9-18a3-488b-a5df-40bfdc9eea29/