The B, E, G, sources and the behavioral R, C, L elements are powerful tools to set up user defined models. Unfortunately debugging these models is not very comfortable.

Example input file with bug (log(-2)):

B source debugging

V1 1 0 1
V2 2 0 -2

E41 4 0 vol = 'V(1)*log(V(2))'

.control
tran 1 1
.endc

.end

The input file given above results in an error message:

Error: -2 out of range for log

In this trivial example, the reason and location for the bug is obvious. However, if you have several equations using behavioral sources, and several occurrences of the log function, then debugging is nearly impossible.

However, if the variable ngdebug (see 17.7) is set (e.g. in file .spiceinit), a more distinctive error message is issued that (after some closer investigation) will reveal the location and value of the buggy parameter.

Detailed error message for input file with bug (log(-2)):

Error: -2 out of range for log
calling PTeval, tree = 
    (v0) * (log (v1))
d / d v0 : log (v1)
d / d v1 : (v0) * ((0.434294) / (v1))
values:   var0 = 1
          var1 = -2

If variable strict_errorhandling (see 17.7) is set, ngspice exits after this message. If not, gmin and source stepping may be started, typically without success.