  Exclusive diffusion on a one-dimensional lattice is studied. In the model
particles hop stochastically into both directions with different rates. At the
ends of the lattice particles are injected and removed. The exact stationary
probability measure is represented in form of a matrix product as a
generalization of the solution given by Derrida et al \cite{dehp} for the fully
asymmetric process. The phase diagram of the current on the infinite lattice is
obtained. Analytic expressions for the current in the different phases are
derived. The model is equivalent to a $XXZ$-Heisenberg chain with a certain
type of boundary terms the ground state of which corresponds to the stationary
solution of the master equation.
