I don't know the Boss DR 880, but as a general rule you should set it as a master slave; then you have two possibilities:
1- the Boss has a "syncro start" command (like all arrangers keyboard): in this case it will start to play as soon as you press a key on the RD 700 SX.
2- the Boss has no "syncro start" command and in this case you have to play one of the internal patterns in the 700 SX to trigger the Boss.
Hope this helps.
_________________________
Korg Kronos 61 and PA3X-Pro76, Roland G-70, BK7-m and Integra 7, Casio PX-5S, Fender Stratocaster with Fralin pickups, Fender Stratocaster with Kinman pickups, vintage Gibson SG standard.