Ngspice supports creation of `User-Defined Node' types. User-Defined Node types allow you to specify nodes that propagate data other than voltages, currents, and digital states. Like digital nodes, User-Defined Nodes use event-driven simulation, but the state value may be an arbitrary data type. A simple example application of User-Defined Nodes is the simulation of a digital signal processing filter algorithm. In this application, each node could assume a real or integer value. More complex applications may define types that involve complex data such as digital data vectors or even non-electronic data.

Ngspice digital simulation is actually implemented as a special case of this User-Defined Node capability where the digital state is defined by a data structure that holds a Boolean logic state and a strength value.