This paper describes a low cost, DSP-based system for real-time control of the tokamak ISTTOK operation, aiming at enlarging the plasma duration (from 40 ms to 1 sec) by AC discharges. This system is based on: (i) An new set of 12, 1 MHz pick-up coils measuring the poloidal field. (ii) An on-site developed analogue active integrator and signal conditioning module (iii) A commercial DSP board and a modified digitizer module; (iv) Specially developed real-time software running on the DSP which controls the acquisition of the signals, implements an fast algorithm for calculation of MHD equilibrium current moments in every millisecond time-window and the and a feedback control routine (e.g. PID), which in turn generates a PWM signal to control the primary transformer current and the vertical field in each sampling cycle. The signals from the pick-up coils are also sampled at a higher rate (12.5 kSamples/sec), saved on the board local RAM memory and transferred to the ISTTOK central acquisition system following the completion of discharge for finer analysis with other equilibrium codes. After laboratory calibration tests, the system has been implemented on the ISTTOK, providing results that are consistent with other installed "off-line" diagnostics and fulfil the real-time constrains.