a modular infusion pump system utilizes a positive displacement pump for pumping a known quantity of fluid for each stroke . a plurality of longitudinally spaced apart fixed combs cooperate with a resilient tube to define a known pumping volume . a plurality of movable combs are interdigitated with the fixed combs and are periodically linearly directed to cause the pump volume to be compressed , and therefore the fluid expelled . a rotary to linear drive system is connected with the movable combs and a ratchet system prevents counterrotation when the motor is in the off position . valves are disposed above and below the pumping unit to appropriately seal off the pumping volume . the valves and the movable combs are operated by means of cams carried by a common shaft .