I tried to start some drumpatterns or songs of the Boss Dr-880 drumcomputer simultan. when I start playing on my Roland RD-700 SX stage piano (which has also large possibilities a a master midi keyboard), but none of both manuals made me clear wether I can do this or not .. Can someone clear my mind i I can do so and if, how I should do that?

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.

Thank you very much for your reply. I just made a quick trial, the Boss Dr-880 has a 4 sync modes:internal; midi (slave mode), remote (only the start/stop operations are controlled by midi messages received from an extrenal midi device) and auto(master/slave settings are switched autom. : it becomes slave when it receives a start message from a external midi device).
So, I suppose the Dr-880 has the syncro start command by means of those settings?
But by by doing so, i still haven't any result, even after ajusting the send the send/receive signals ..? Nothings starts when pressing any key on the 700SX ...

In that case you can control the start function on the Boss playing one of the internal styles in the RD 700 SX (I know that there are, just like the old RD 700 had them...)

Careful here. There are two different things being discussed.

The "internal; midi (slave mode), remote (only the start/stop operations are controlled by midi messages received from an extrenal midi device) and auto(master/slave settings are switched autom. : it becomes slave when it receives a start message from a external midi device)." stuff refers to the way the Boss will react to external clock and start/stop messages.

This is nothing to do with "syncro start" as found on arranger keyboards, which I think is the feature you are looking for.

I don't think the Boss will auto-start in response to a keyboard depression on the master keyboard. To start the Boss from the master keyboard you will have to explicitly send a "Song Start" message.

Could start/stop the Boss using a footswitch? At least this would free up your hands!

Indeed, the 700SX has them and I tried also that suggestion yesterday out, but no start at all ... I will overlook the midi settings on both once more this evening and have a further look at it. Thanks once more dreamer!

Well MacAllcock that's what I'm intending to do as the last and most facilitating solution. But as I'm a little bit perseverant in my intentions [:-)] (what other should be the 'existing-reason' of midi)i would be glad to find the solution for my problem, if there is one at least.