The `digital' node type is directly built into the simulator. 12 digital node values are available. They are described by a two character string (the state/strength token). The first character (0, 1, or U) gives the state of the node (logic zero, logic one, or unknown logic state). The second character (s, r, z, u) gives the "strength" of the logic state (strong, resistive, hi-impedance, or undetermined). So these are the values we have: 0s, 1s, Us, 0r, 1r, Ur, 0z, 1z, Uz, 0u, 1u, Uu.