A microprocessor-based firing control scheme for the constant firing angle of the thyristors of the line commutated converter under a variable frequency source is presented. The generation of triggering pulses and the control of the firing angle are performed by microcomputer software. Linear on-line firing angle control from 0° to 180° is achieved satisfactorily. The firing angle, once adjusted, remains constant irrespective of the changes in a.c. supply frequency. The scheme is also capable of measuring on-line supply frequency without any extra hardware. The firing control scheme is implemented using the Intel 8085A microprocessor-based system.