The first unit is connectable to a first voltage source and {HEAD ~ the second unit} is connectable to {TAIL ~ a second voltage source} 
is connectable to
The first unit is connectable to a first voltage source and { HEAD ~ the second unit } is connectable to { TAIL ~ a second voltage source }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL