During operations with a manipulator arm situations will occur when the manipulator end-effector is, or comes, in contact with its environment. The dynamics equations, which describe the motion of the manipulator, can be represented by an ordinary differential equation for the free motion and an additional algebraic (in)equality for the motion constraint. At the Dutch National Aerospace Laboratory, NLR, recent work has led to a new formulation of the dynamics equations for constrained motion. Subsequently from this formulation a discretization for the numerical integration of the dynamics equations was obtained which is stable with respect to error accumulation in the constraint. In the present study this formulation and discretization is adopted for constrained motion, based on an elementary collision model: - the manipulator end-effector is confined to a rigid, frictionless and flat surface. The motion constraint is then specified by an algebraic equality. - the manipulator end-effector comes into contact with a rigid, frictionless and flat surface. The motion constraint is now specified by an algebraic inequality. The collision between the end-effector and the constrained surface is assumed inelastic and frictionless. Simulation studies are performed with the aid of the Machine Independent Model Oriented Simulation Environment (MIMOSE) available at NLR. As basis for the simulation model an existing model of the Hermes Robot Arm, as developed for the HERA Simulation Facility Pilot (HSF-P) is used. These are the already realised parts of the NLR Information System to support the development of digital simulation models for Multibody Systems (ISMuS). From this study can be concluded that the applied method to treat equality constraints shows good results in relation with the simulation of constrained HERA motion.