On Mon, Feb 3, 2025 at 08:26 PM, eewiz wrote:
Is there any good reason to plumb the ground net (node 0) out of a hierarchical schematic block?
Inside the block, the hierarchical label could be called RTN and outside the block, the hierarchical label could be connected to ground (node 0).
Assuming it is always connected to node 0 outside the block, it appears to me that there is no good reason to do the plumbing.
There are actually some "tricks" going on, when a subcircuit (or lower-level schematic) has a "ground" net that comes out on a .subckt pin.? Some years ago I spent some time documenting this, but did not finish.
?
When a subcircuit has its "ground" pin brought out to the higher-level, and if that pin is not connected to node 0 / ground at the higher level, then the internal "ground" is no longer ground.? It becomes whatever that pin connects it to.
?
This is particularly useful when you inherit a subcircuit model from someone else, which has internal points connected to ground, but you don't want to use it that way.? Say that you have an IC that is "normally" powered by +5 V and ground.? The guy who made the SPICE model might have written it so that the "ground" pin goes to node 0 inside their subcircuit model.? But you might want to use it powered by +2.5 and -2.5 V.? In the old days, what we had to do was re-write the vendor's SPICE model, changing all their node 0 netnames to something else, so that it could be NOT connected to ground.? What LTspice allows you to do, is just bring that net out on a pin, letting you use it as if it was not node 0.
?
This looks wrong, because the lower-level schematic (or .SUBCKT block) seems to be using node 0, but it actually isn't.? LTspice handles the net name translation automagically.
?
By the way, I am not sure if everyone here understands your use of the words "plumb" or "plumb out".? Honestly, I am not sure that I know exactly what you mean either.? Remember that a sizable percentage of?the members of this group are not in North America and do not know our slang.
?
Node 0 is the only universal node.
Except when it isn't.
?
If I remember correctly, LTspice uses "GND" as an alias for node 0, but not "ground", and not nodes 00 or 000.? The fact that node "ground" is not the ground node (node 0) can be a little confusing, because LTspice displays that both of them are ground if you hover over them, but one is "ground" while the other is "node ground".
?
Andy
?
