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PCB schematic capture basics (part-1)
#1
Hi Guys

Using computer software to design printed circuit boards traditionally involves two steps: capturing the schematic, then laying out the board. Technically there is a third step that precedes these which is to make the library of parts, but we will assume here that a device library already exists. In which case, you could still say there is a third step afterwards, which is to generate the Gerber and Excellon files for the PCB house to use to manufacture the board.

Schematic capture is actually YOU manually drawing the schematic in the "schematic editor". 

I've only used Eagle (and Maxi-PC back in 1987) but I would think most PCB design software is more similar than different? In Eagle, you go to the File button, then select New from the drop-down menu, then select Schematic in the second drop-down menu. The schematic editor will open.

Across the top of the window are three tool bars: The top one has file management menus as File, Edit, Draw, View, Tools, Library, Options, Window, Help. Some of these functions are duplicated as icons in the second toolbar, as: folder, Save, Print, Tile, Board/schematic-jog, page sheet-number-window, Use, Script, Run, Zoom functions (full, in, out, redraw) Undo, Redo, Stop, traffic-lights. The third toolbar has just the GRID icon, which is a group of dots (?) and is very important.

For schematic capture, the GRID MUST BE SET TO 0.1" or the connections to the parts will not be made. Right click on the scale icon and the GRID window opens. You see Display on/off which provides grid-lines that are visible or not. Beside this is Style Dots/Lines, where the visible grid indication can be solid lines or lines of dots. It is normal on the schematic to turn the display OFF, in which case it does not matter which style is selected.. Below this is the Size window, where you enter 0.1 and then make sure the default "inch" is visible
Below this is a window called Multiple, which is usually set to 1 (one) and then the Alt window which might be defaulted to 0.01. These are noncritical. Press OK to set the grid.
the scale, zoom functions, 

Down the left-hand side is a tool bar with all the functions available for the schematic editor, in two columns of icons. One of them is the Add function, which looks like a logic AND gate, which is how you access the library of parts. Left-click and a window named ADD opens showing a list of libraries. Scroll through the list. If you know which library has the part you want, left-click on it and it will open and you can see all the parts in that library. Scroll through it and left-click on the part you need. When you do this, the schematic SYMBOL will appear in one of the right-hand boxes within the ADD window, and the PACKAGE will appear in the box beside it. If this is the correct part, then click on OK and the ADD window disappears and the part is now stuck to the cursor in the schematic editor.

Use the mouse to move the part anywhere on the screen you wish, then left click to drop the part. The virtual part is still attached to the cursor allowing you to drop as many of this components onto the schematic as you wish. This is handy when you need a lot of resistors, for example, or capacitors, or any other component. Each part dropped onto the schematic will be assigned a geographic NAME in the sequence of their addition, so if resistors, they will be R1, R2, R3, and so on.

You can MOVE the parts around using the MOVE icon, which is like a compass with an arrow pointing up (north). Left-click on MOVE then cursor to the part you want to move, left click on the part and it is attached to the cursor. If you right-click the part will rotate and stay on the cursor until you left-click again to drop it in place. Each rotational step is 90-degrees, which is appropriate for the schematic as all the connection points on the component, called PINS, must be on the 0.1" grid which is square. 

To connect the components to each other, we draw NETS between them. The NET icon has a hatch-mark line at top that bends and goes down to the right, with three horizontal green lines within. Left-click on this and the third tool-bar along the top will now have options for the wires that you will draw. The first are how the wires can bend, with common default being 90-degrees. The next is orthogonal (45-degrees), then random, and curving options, then the mitre-window, and infill-windows, then Style (Continuous, LongDash, ShortDash, DashDot), and the Net Class ((0-default). Generally, we use the defaults of 90-degree bends, no infill, continuous and 0-default class.

After clicking on the NET icon, we cursor to the pin of one component, then right click, then cursor up or down or sideways as appropriate. There will be a green wire attached to the cursor that will bend where we click until we reach another component pin. We can cursor to any point along the wire we have just laid and make a branching wiring to another component. A dot will appear at the junction when we make the next click on the branch, at a bend or at the next component pin.

Each group of component ends that are tied together is called a "net", short for "network". Each net is assigned a name by Eagle, as N$1, N$2, N$3, and so on. The name of the net is inconsequential to us most of the time and never see these names anyway unless we select the Information icon, or have a reason to rename the nets. We will discuss these things later.

The first time you save the schematic you will be asked where it should be saved and to name the schematic. This is mostly a computer file management issue, following the tree to a point within Eagle where the schematic should be accessible. Eagle has a Projects folder, which is where these things should go. It is a good practice to make a new project folder for your new schematic because you will eventually have a related board and the CAM files needed for manufacturing. That is twelve (12) files overall for the one board, so best to isolate it from your other projects with each in its own project folder.

As you go along adding parts to the schematic, SAVE FREQUENTLY. Eagle likes to crash especially if the computer is older or has been on for a few hours and it would be frustrating to lose hours of work.
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#2
Schematic capture Part-2

Very soon after you start the schematic you should SAVE it.
If you simply press SAVE, the Windows file management window will open with a "Save in" command line and a window below showing the recent files saved. Click on the scroll-arrow for the Save-In window, and select "EAGLE". A revised window opens with a list of files within Eagle; scroll down to "Projects"Left-click; press OPEN then the list changes to show the Project folders within Eagle.

If this new schematic should go into a new project folder, left-lick on the Create-New-Folder icon and the new folder will appear at the end of the Project folder list with its name high-lighted for you to enter a new name (other than "new folder". Type in the name of the folder and press ENTER (RETURN).If you now look at the Project list in Eagle, the new folder will be in its alphabetical position. Left-click on the folder and see the schematic listed there. Open the schematic and continue working it.

You save a few keystrokes by going to File, then selecting "Save as".

From here on, when you work on the schematic and pres SAVE, the file is invisibly saved and you can keep working.
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#3
Schematic Capture and Board Layout

In Eagle, as you build the schematic you need a bit more space as more components are added, and as you determine that circuit sections should be moved apart to make it easier to read. Similarly, as you work on the board in the Board editor, you will be constantly zooming in and out to see things close up or to see the bigger picture.

All of this zooming in and out and resizing will make Eagle "tired", and subsequent zooms may not allow the entire circuit or board to be visible. At this point you need to FIT the view to the window using the FIT option in the section of ZOOM parameters in the upper toolbar. FIT is a magnifying glass with a white centre. Left-click on this and the view of the schematic or board will snap to fit within the window. It is anew starting point for zooming in and out that allows everything to work more as it should.

Remember to FIT the view regularly to avoid a program crash.
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