Cable robots or wire driven robots possess many advantages that make them very well suited to be used as haptic interfaces. They exhibit very low inertia and very low friction because of their very light mechanical structure. The use of cables, however, leads to an under-constrained system which shows complex properties. The main drawback is the difficulty to estimate the useful workspace, and the maximum tension in the cables, as the relation between the maximum force at the tip of the interface and the maximum tension in the cables can not be easily established. In this paper we present a method to calculate these tensions in a given workspace and to estimate what we have called "the tension capable workspace" for a 4 cables 3 d.o.f (degree of freedom) haptic interface. We then use this method to design a new singularity free architecture and compare it to more "classical" architectures in terms of workspace and maximum tension.