kicad-developers team mailing list archive

[Wayne Stambaugh <stambaughw@xxxxxxxxx>]

On 4/20/2016 4:00 PM, Simon Richter wrote:
> Hi,
> 
> as wxWidgets is getting on my nerves with editing widgets in the pin
> table not rendering properly, I've started on support for net ties.
> 
> In the current iteration, they would be placed the same way as junctions.
> 
> Rules:
> 
>  - Any wire or pin connected to a net tie is in a separate net (unless
> connected elsewhere).
>  - The net tie maps to a pseudo-pad that all three nets need to be
> connected to.
>  - Connecting the nets there does not give a DRC error -- anywhere else
> will.
>  - The pseudo-pad can be placed on a regular pad if it is on one of the
> nets connected to the net tie.
> 
> Use cases:
> 
>  - Analog and digital supply planes connected with a trace, but
> otherwise separate

I'm going to put my EE hat on and say that if you connect two power
planes with a trace then they are the same plane no matter what you
called them in your schematic.  A more typical solution in this case
would be to physically separate them by some type of component or
components.  Usually inductors or 0 ohm resistors (aka jumpers) are used
in this situation depending on what you are trying to accomplish.

>  - Current sense resistors between a supply rail and a load
>  - Decoupling capacitors.

I can see the decoupling capacitor use case where you want to tie a cap
to a specific component power pin.

> 
> I've added UI[1] and save support in eeschema already, still needs
> mapping to the netlist and pcbnew support.

Are you aware that changes to the current schematic file format are
forbidden until we (I) finish implementing the new file format?  This
was discussed fairly recently so everyone should be aware of this.  In
any event, you should have gotten input from the development team before
heading down this path.  This is good advice for any developer.  Even I
solicit input on new features or large changes because other devs always
seem to think of things I didn't.

I don't have a strong opinion one way or the other about this feature.
On the surface it does seem useful but I've never seen any EDA product
support this so board designers may not understand why they would want
to use it.  Any one else have any thoughts on this?  You may also want
to check with the users to see if it's something that they would even use.

> 
> There doesn't appear to be a real standard on how to represent net ties
> in the schematic, though. A design note[2] from Linear Technologies uses
> 45 degree angles on wires to make it look really intentional that the
> wires should meet in the same spot, but that would be a major hassle
> both to implement and use.
> 
> For now I've gone with a larger dot, but that is very unintuitive.
> Printing net names next to wires is difficult, because these are still
> wires only. Numbers next to the wires might be doable, but confusing, so
> if anyone has a good idea how to represent them, please speak up.

How about a different color dot or a different shape.  A different shape
may be better for users who are color blind.

> 
>    Simon
> 
> [1] http://psi5.com/~geier/net-tie.ogv
> [2] http://cds.linear.com/docs/en/design-note/dn434f.pdf
> 
> 
> 
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