The circuit description

One element per line. The first character of the name picks the type; the fields that follow are positional and comma-separated.

e1,1,0,12
r1,1,2,1'k
r2,2,0,2'k

That is a 12 V source across a 1 kΩ and a 2 kΩ in series. Node 0 is ground, always, and every circuit needs one.

The fifteen letters

name is always the first field. Everything else is positional.

Italic fields are optional. The initial conditions on l and c are read by FD and TR and ignored by DC and AC.

Note what m takes: the names of two inductors, not nodes. Node order on those inductors is the dot convention — the first node you name in each coupled element is its dotted end. Get it backwards and every current comes back wrong, looking right.

An m has no answers of its own. It is a statement about two other elements, and its effect shows up in their currents.

Names

Letters, digits and underscores. The first character sets the type, so rload is a resistor and r_b is a resistor.

A name must be identifier-safe. r-x looks reasonable and is refused, because 2*i_r-x would silently read as 2*i_r minus x.

Node names are yours: 1, 2, in, out, b, e, c. Only 0 is reserved.

Values

A value is a number, a symbol, or an expression in either.

e1,1,0,vs
r1,1,2,1'k
r2,2,0,1'k
e2,3,0,5*v2
r3,3,0,2'k

vs is a symbol — every answer comes back in terms of it. e2 is a dependent source: its value names v2, the voltage at node 2, so it is five times whatever the rest of the circuit puts there. Any answer name may appear in any value. That is how all four dependent-source types are written, and why there is no separate letter for them.

An underscore in a symbol makes a subscript in the typeset answer, and nothing else. Underscores, and subscripts in your answers has it.

SI prefixes

An apostrophe and a letter multiply the value, exactly:

4.7'k · 10'u · 560'm · 1'M

Eleven prefixes, peta down to atto; the full table is in “SI prefixes”. Two things to know here: case matters — 'm is milli and 'M is mega — and a prefixed value is exact, as is a plain integer. 8000 and 8'k are exact; 8000. and 8E3 are approximate, and that difference shows up the moment an answer is symbolic.

Brackets

Brackets mean exactly three things.

A parallel group, in a resistor's value only. [r1,r2,r3] is those three in parallel:

rp,1,0,[1'k,2'k,2'k]

A pair of terminals, for a transformer or a two-port, when neither side sits on ground. [top,bottom]:

e,1,0,10
r1,1,a,50
r2,b,0,50
z1,[a,b],[c,d],[100,10,20,50]
rl,c,d,1'k

Neither port of z1 touches ground here. The two-node form z1,1,2 is the same element with both bottoms on 0, and a four-terminal block reports a current at each of its four terminals rather than two.

A parameter set, the four numbers of a two-port, or a transformer's two turns counts.

Anywhere else, a bracket is an error rather than a guess.

Separators

A line each is the readable form. A colon does the same job on one line — e,1,0,12:r1,1,2,1'k — and both are accepted everywhere, including in a saved file.