The article suggests a method of transition from continuous models of automatic control systems (ACS) to discrete ones, based on the application of quadrature formulas. This makes it possible to obtain a model of ACS in the form of a system of linear algebraic equations instead of differential equations. The result of this solution is the discrete values of the ACS state coordinates. That allows one to reduce the expenses of microprocessor time on that part of algorithm of the task which is responsible for the process of calculation of a system condition. The state of the control object and the control law are stored in the controller memory in the form of numeric arrays which is rational when transferring to the digital control of continuous ACS.