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In order to increase the performances and decrease the size and cost of control circuits for pulsewidth modulated ( PWM) inverters, a microcomputer control system has been developed. It consists of a microcomputer and an I/O processing unit large-scale integration (LSI) to control a variable-speed motor. The microcomputer performs complicated calculations and analyses of driving conditions. The I/O processing unit LSI detects motor driving states (analog to digital convsrsion and pulse count, etc.), generates PWM pulse outputs, and protects the inverter against overcurrents and power outages. The I/O processing unit LSI functions using the command data from the microcomputer. Conversion and count data can be sent to the microcomputer from the I/O processing unit LSI. For the convenience of interfaces to the microcomputer, all-digital logic circuits are developed for the I/O processing unit LSI. For example, PWM pulse outputs are generated by comparing digital data of a carrier timer and step level of a modulation wave. Undesirable harmonic components can be reduced easily by command data from the microcomputer.