Synchrotron control system of HIMAC

HIMAC, Heavy Ion Medical Accelerator in Chiba, has been in operation for about 8 years. The control system of HIMAC synchrotron is now undergoing renewal and upgrade. As a medical accelerator in use, reliability and availability of the beam must not be compromised in HIMAC. We decided to change most computers of the system in a node-by-node way, over a couple of years. Among the replaced are: Power-supply Controllers, RF-system controller, Beam Transport, and Timing System controllers. The steps taken and to be taken are reported.