DIAGNOSTIC SYSTEM FOR ERROR DETECTING CIRCUIT

PURPOSE:To diagnose an error detecting circuit easily with a low cost, by inputting normal data from an external device and changing this data for a constant time to diagnose the error detecting circuit. CONSTITUTION:When an FF8 which indicates the diagnostic operation is set by a control part 11 and data is read from a floppy-disc driving FDD1 to an LSI2 for floppy-disc control, a transfer request is generated from the LSI2 to set an FF10. An output O of the FF10 becomes low, and the input of separate data from the FDD1 to the LSI2 is inhibited by a gate 5. Next, when an FF9 is set and the FF8 is reset and a separate clock is inputted from the FDD1, the FF10 is reset through a gate 7, and the inhibition of the gate 5 is released to input data from the FDD1 to the LSI2. By this action, normal data has one bit dropped out and is inputted to the LSI2 from the FDD1, and the LSI2 reports a CRC error, thus diagnosing the error detecting function.