A combinatorial drug delivery device comprising: a plurality of serially connectable modules, each of the modules including at least one drug component; and, a master controller having a source of power and a ground, characterized in that, each of the modules includes: a power line, wherein, with the modules being serially connected, the power lines are connected in series between the modules with the serially connected power lines being connected to the source of power; a ground line, wherein, with the modules being serially connected, the ground lines are connected in series between the modules with the serially connected ground lines being connected to the ground; a multiplexer having a plurality of identified input channels (3a, 3b, 3c, 3d) and a single output, wherein at least a subset of the input channels is selectively connected to one of the power line and the ground line of the corresponding module; a first digital logic line configured to select the input channels, wherein, with the modules being serially connected, the first digital logic lines are connected in series between the modules with the serially connected first logic lines being connected to the master controller; a voltage reference line having a first resistor of known value in line with the output of the multiplexer, and a second resistor of known value in parallel to the first resistor, wherein, with the modules being serially connected, the voltage reference lines are connected in series between the modules with the serially connected voltage reference lines being connected to the master controller; a branch line connected to the voltage reference line having a third resistor of known value and a normally closed switch thereon to connect the branch line to the ground line, the switch being opened with connection to a further module; wherein, the master controller, using the first digital logic line, selects, in sequence, the same identified input channels across all of the modules with measuring a reference voltage across all of the modules for each of the same identified input channels. a plurality of serially connectable modules, each of the modules including at least one drug component; and, a master controller having a source of power and a ground, a power line, wherein, with the modules being serially connected, the power lines are connected in series between the modules with the serially connected power lines being connected to the source of power; a ground line, wherein, with the modules being serially connected, the ground lines are connected in series between the modules with the serially connected ground lines being connected to the ground; a multiplexer having a plurality of identified input channels (3a, 3b, 3c, 3d) and a single output, wherein at least a subset of the input channels is selectively connected to one of the power line and the a first digital logic line configured to select the input channels, wherein, with the modules being serially connected, the first digital logic lines are connected in series between the modules with the serially connected first logic lines being connected to the master controller; a voltage reference line having a first resistor of known value in line with the output of the multiplexer, and a second resistor of known value in parallel to the first resistor, wherein, with the modules being serially connected, the voltage reference lines are connected in series between the modules with the serially connected voltage reference lines being connected to the master controller; a branch line connected to the voltage reference line having a third resistor of known value and a normally closed switch thereon to connect the branch line to the ground line, the switch being opened with connection to a further module; wherein, the master controller, using the first digital logic line, selects, in sequence, the same identified input channels across all of the modules with measuring a reference voltage across all of the modules for each of the same identified input channels.