This paper presents an open architecture robot control system for lower level manipulator control such as motion control or force control. Basically, the system consists of three elements: an industrial robot manipulator called A-HAND, a servo computer with the motor driver units, and a host computer. The system is called ARS/A (Aoba Robot System for A-HAND). The robot and the servo computer are regarded as an independent robotic module with a standard interface to the host computer, from which it accepts a set of real time commands to control the robot. Any computer having an interface may be connected to the robotic module as a host computer. To design the set of real time commands is a crucial issue because it determines the capability and flexibility of the robot system. This paper proposes a set of real time commands which are needed for lower level control experiments. The set was found through trials. A real time monitor called MOS/A (Motor Operating System for A-HAND) to process the commands to control the robot are implemented on the servo computer. The MOS commands are defined as functions of a C language on the host computer. The C language is called ARC/A (Aoba Robot C Language for A-HAND) to have other robot control utility functions such as graphic simulation functions as well as the MOS functions. Sample programs show that ARC/A is an efficient programming tool for lower level control.