Ratsnests

Ratsnet lines show the Pin to Pin connections from the loaded netlist. When “real” connections are made with routing, the ratsnest line associated with that connection will be removed, or redrawn if there are any remaining connections. Copper Shapes will also remove any ratsnests associated with them once they are assigned to that netname.  Display of the ratsnests can be useful in placement but, especially for more dense designs, the display of the ratsnests can be a distraction. There are toolbar icons to display, or blank, all of the ratsnests and menu entries to control ratsnest display for selected Components and Nets. Note that the ratsnests will need to be blanked before any limited selection is shown.

Swapping Components, Pins and Functions

Swapping of Components, Pins and Functions (“gates”) can help to “unravel” the ratsnests. Swapping of Components needs no setup, swapping of Pins needs to be enabled with the Pin Group property in the Schematic Parts and swapping of Functions (“gates”) is automatically enabled for any multi-section Schematic Parts that are identical. Whilst Pin and Gate swapping in the PCB used to be a common practice in designs using small scale integrated circuits, this is less applicable to designs using more modern, complex, parts.

Alternate Symbols

In some designs there may be a variety of possible footprints, or assembly options, for a component that can only be determined once the PCB has been laid out, or even after a prototype has been built. To allow for this, the ALT_SYMBOL property can be added to the Schematic Part to define any alternate symbols using a comma separated list, and the alternate symbol selected during placement, without any change to the Schematic, or Bill of Materials.

Note that the lab example is somewhat artificial since the original DIP and alternate SOIC would have different Part Numbers and would require changes to the schematic and BOM to be of any use. A good example of this might be having the IPC 7351 Minimum, Nominal and Maximum footprints defined so that during placement you can choose which footprint to use, based on space requirements on your design.

Placing from the Schematic

In the previous lab for placement, the entire design was placed from within PCB Editor but it is also possible to place parts by selecting parts in the schematic with Intertool Communication enabled in OrCAD Capture, the default, and PCB Editor running the Place>Components Manually command.

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