Abstract: This paper deals with a closed-loop digital control for a hydraulic valve, controlled motor speed system. In the system, a BITBUS microcomputer is used for PI controller. In stead of the conventional scheme of tachometer generator plus A/D, a digital speed test is adopted, and a full digital speed feedback control is implemented. An IBM-PC microcomputer can communicate with the BITBUS, therefore, the process monitoring for the system can be performed on the IBM-PC microcomputer. So the system possesses greater reliability at a given cost, operating flexibility, improved data display, and so on. Through sufficient experiments, parameters of the PI controller have been determined and better control properties have been achieved.