Doepfer LMK2+ Midi Master Keyboard (old version without USB, no longer available) - Manual (Page 12)

LMK2+ User's Guide Page 12
Appendix B: Pin Assignment of the Jack Sockets
"high"
end
"low"
end
2...10k linear
Foot Controller
("Volume")
Foot Controller
("Sustain")
Foot Switch 2 (Sostenuto)
Foot Switch 1 (Sustain)
1 2
Power Supply
1 = +7...12V DC
2 = GND