The M6R7 was the basis for most older drive designs and is still used on older systems with 5 lead motors, or very high voltage (24-30VDC) motors. It allows for full/half step operation with a dropping resistor between the supply voltage and motor to limit the current. While the L/R technique is simple and inexpensive, it is also inefficient due to the large power losses from the dropping resistor. It is typically used in older systems as a replacement. A power supply is required, and purchased separately. See chart below.