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Is there any difference between the following two netlists for a resistor connected between node5 and node0 with resistance 20K ohms?

• R1 5 0 20k

• R1 0 5 20k

If not, is a resistor the only type of component that is always symmetrical, or are there other components as well where the order of the connected nodes does not matter? Additionally, is it possible in a netlist to use a node name instead of a number (for example set 0='ground')?

Examples taken from: https://www.cpp.edu/~prnelson/courses/ece220/220-spice-notes.pdf

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  • \$\begingroup\$ I've not seen a netlist like that. I would expect something more like [netname1, R1 1, node5]. which says "connect pin 1 of R1 to node5" and [netname2, R1 2, node0]. \$\endgroup\$
    – SteveSh
    Commented Feb 11, 2020 at 0:08
  • \$\begingroup\$ @SteveSh -- thanks I've just updated the question with a reference of where I found them from. Also, what is a netname? Is that a particular 'section' of the circuit? \$\endgroup\$
    – David542
    Commented Feb 11, 2020 at 0:11
  • \$\begingroup\$ I see, this is a Spice netlist. I was thinking of something for building a printed wiring board from a schematic. \$\endgroup\$
    – SteveSh
    Commented Feb 11, 2020 at 0:15
  • \$\begingroup\$ A net is any connection in a circuit. A net can have any number of connections. Vcc is a common net name for the power supply network. GND is another net name with many connection points. In a schematic, the designer give nets meaningful names (like "input", "out1",...), or allow the program to assign default net names (usually something like "N001, N002..." \$\endgroup\$
    – Peter Bennett
    Commented Feb 11, 2020 at 0:36
  • \$\begingroup\$ @David542 - netname is just a name given to a connection, or a NET on a schematic and in the netlist for the printed wiring board. At the schematic level, netnames can be assigned by the designer, such as CLK_50MHz, or can be automatically assigned by the schematic capture tool, in which case they have mysterious names like what Peter Bennett alluded to, \$\endgroup\$
    – SteveSh
    Commented Feb 11, 2020 at 15:00

3 Answers 3

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Generally, the pin order matters to the model since the order defines what connection goes where. SPICE can tell the difference and treats them differently. However, if within the model it is symmetrical such that the pins are interchangeable, then the external result is manifested as the order not mattering.

So technically SPICE is treating the pins as unique and it is processing it differently in the model depending on which way it is connected, but way the numbers end up crunching out in the model, there is no difference in the end result (for symmetrical components at least. Obviously this isn't the case for something like a diode).

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  • \$\begingroup\$ thanks, are there any other symmetrical components other than a resistor? \$\endgroup\$
    – David542
    Commented Feb 11, 2020 at 0:30
  • \$\begingroup\$ @David542 inductors, some capacitors, crystals \$\endgroup\$
    – DKNguyen
    Commented Feb 11, 2020 at 3:17
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There is no difference. In the end, you have a 20 Kohm resistor between two nodes (net points).

If this was a part that was not symmetrical in so far a current flow goes, like a diode, then I think you would get different operation/function from the two configurations.

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Unless the current through it is referenced somewhere in the circuit, is doesn't matter. That is because the current through 2-pin elements goes from pin #1 to pin #2, which means the sign changes. Inductors and capacitors are in this same category. Sources and diodes need obvious polarity considerations.

GND is meant to mean 0 (zero) when used as a net label. GROUND can be safely used to not mean GND while providing an external grounding for the symbol.

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