As an exercise, let’s break it down:
The Model: Cycles
The M:C has two ways to get signal out. “MAIN OUT” and “HEADPHONES”.
“MAIN OUT” has a separate jack for L and R, so we know each is mono. If we want to send a both to another device, we’ll need two cables.
But what kind of cable? We don’t know yet if these ports are balanced or unbalanced. Checking out the manual, it says MAIN OUT is balanced if we use TRS cables but unbalanced if we use TS:
This is the normal behavior of balanced outputs, by the way. But it’s really nice of the manual to spell it out for us like this! Elektron documentation is good.
HEADPHONES has only a single jack. Is that use for mono or stereo signal? Because it’s for headphones, we could assume stereo, but we check the manual to be sure:
Yup. Stereo. Stereo over a single cable means we have to use TRS so we can use the T for the left and R for the right.
The SP-404mkII
Now let’s look at the 404mkII. It has a LINE IN that has an L and an R jack. We can therefore assume each expects a mono signal.
But does it want the signal balanced or unbalanced? Roland documentation is the worst, but we can tell they want unbalanced because it doesn’t mention anything about balance for the LINE IN but does for the LINE OUT:
Putting Them Together
Seeing as the 404 has a separate L and R jack for its LINE IN, and the M:C has separate L and R jacks its MAIN OUT, the easiest thing would be to just connect the two using two cables. What kind of cables? Well the 404 suppors TS, and the M:C supports either TRS or TS. The lowest common denominator there is TS, so I’d use that (though, in practice, if you only had TRS cables around it probably wouldn’t hurt anything).
What if we wanted to use the HEADPHONE output of the M:C instead? Well, now we’ve got both L and R on one TRS cable, and the 404 is expecting L and R over individual jacks. So we’d have to split the signal before it got to the 404 with a “breakout cable” like this:
It just takes the T and R of a TRS and splits it into two separate TS.
And that pretty much covers all the possibilities. If I’ve overlooked anything, please ask about it. I think breaking down examples like this is invaluable to picking this stuff up.
s and 
s IMHO 
