A pulse oximeter with drive lines for driving red and IR LEDs, and a drive
circuit for driving those drive lines. A processor controls the drive
circuit using a red zero output line and an IR zero output line directly
connected between the processor and the drive circuit. This allows a
control signal to directly control the turning off of either the red or
IR drive transistors to prevent forward current flow through the red and
IR LEDs by overriding the ongoing programmable logic state machine
control of the drive transistors. The effects of crosstalk and capacitive
coupling are reduced as a result.