6.4.1.1 Overview

Forward annotation is the process of updating a layout to reflect changes made in the schematic. This process is used when, for example, a new component is added to a schematic and needs to be included in the layout. This section describes how to forward annotate changes in a Lepton schematic to a PADS PowerPCB layout.

PADS implements forward annotation through the use of an ECO (Engineering Change Order) file. The ECO file describes the differences between a current design and the desired design. PADS generates the ECO file by performing a netlist comparison between a new netlist file and the netlist contained in the current layout.

6.4.1.2 Detailed forward annotation procedure

This procedure assumes you have a board layout open in PADS and that you have made your schematic changes in lepton-schematic. For the purposes of illustration, assume your schematic is split into two pages in the files pg1.sch and pg2.sch.

• Create an updated PADS netlist by running:

  lepton-netlist -g pads -o mynet.asc pg1.sch pg2.sch

  This will create the netlist file mynet.asc.

• Make a backup copy of your PADS layout in case things fail in a destructive way.
• From within PADS, choose the Tools → Compare Netlist menu item and choose the following options in the form:

original design to compare	use current PCB design
new design with changes	mynet.asc
√	generate differences report
√	generate eco file
comparison options
√	compare only ECO registered parts
attribute comparison level
√	ignore all attributes

Click the ‘OK’ button to create the ECO file.

• Examine the ECO file to make sure it looks ok (the ECO file is a text file which can be viewed with any text editor).
• From within PADS, choose the File → Import... menu item. Locate and choose the ECO file created previously.