From left to right: standard Boss barrel connectors for 9 volts DC, negative tip, in and out; Effect Remote on/off footswitch, Pad control input, Trigger control input, Keyboard control input, Tape control input on RCA phono, Audio Out on phone and RCA phono, Level switch, Audio In on phone and RCA phono.
The Pad input detects a trigger to start the sample playback, as well as a volume level for that trigger by sending a CV to the compander circuit.
The Trigger input just detects a sample start trigger. Volume is fixed.
There are two pitch control inputs, Keyboard and Tape. Tape won't work without Keyboard also being plugged in - with nothing plugged into the keyboard jack, the pitch of the sample playback is determined by the front panel pot. So how does it detect pitch? There's no midi, and it doesn't use a pitch CV in the sense that a modular or analogue synth does. A chip known as a PLL (phase-locked loop) changes the frequency of its VCO when it detects the frequency of the Keyboard signal. To do this, it needs to see a fairly clean, pitch stable, monophonic waveform with minimal harmonics, preferably a sine wave. The Keyboard input also triggers the start of the sample, the volume, and uniquely, the playback time (gate).
Roland had two purposes in mind for the Tape input. The first is a rather complicated way of using a cassette as a backup device for the sampler and it's settings, by recording the initial key pitch, followed by the sample itself, onto different channels of a standard stereo cassette tape, while switching settings on the front of the machine*. Secondly, under the heading "As a Sequencer", there is this single obscure paragraph in the instruction manual:
"By recording the keyboard sound onto the cassette deck and feeding the playback sound to the RSD-10 through the Pitch Control Tape input jack, the sampled sound will be automatically played back".
I'm assuming that the "sequencer" refers to the sequence of pitches on a tape. Of course, any suitable audio source would work here.
This machine, like so many other Roland and Boss effects from this era, is based on the "long chip", the custom RDD63H101 CMOS gate array. But it is the external control circuitry of this machine that enables the magic to happen. It is a fascinating little bit of eighties technology, another example of Roland ingenuity.
* I've never attempted this, however, there is of course a way of doing the same thing with a modern DAW that works quite reliably. You put the audio tone on one track, and the sample on another, and make sure your triggering audio tone starts bang on with the sample you are loading (this works in Mode B Manual Rec/Play).