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Incremental vs. Complete Schematic Capture
#1
When you are considering how to lay out a new project, you often visualise the finished product and work backwards. You actually envision ALL aspects of the product at once and can see the shape of the chassis, where the major components will be positioned on the chassis and on the PCB, where the controls and jacks will be, how the wiring might flow. For the purpose of this discussion, we are assuming a PCB will be the main component carrier. Using a PCB means the design work is "front-loaded" and that making a second, or third, or one-hundredth copy of the product will be very quick to do. if we were hand-wiring, every unit would require the same amount of work even if we are following a fairly specific layout. The wiring time for the PCB amp is usually much shorter than for the hand-wired amp.

Note that on the scale we are working on, ALL the work is by hand: hand stuffing of the PCB: hand soldering; hand assembly; hand wiring of whatever requires wiring; hand testing.

Suppose the project is a simple guitar amplifier.

If the amp is solid-state, then there are not too many thermal considerations other than where the output devices might be bolted to the chassis, or to a heatsink, and if the heatsinking will be adequate?

If the amp uses vacuum tubes, then we have to assure air flow around the tubes and try to keep their heat away from the PCB. Do we put the tubes on the PCB? Do we use chassis-mounted tube sockets and wire to the board from there? Let's suppose we decide to mount the tubes on the PCB. This has many advantages in that the wiring around the tube socket is "locked in" and parasitic capacitance will be controlled and will be consistent for every copy of this amplifier. Besides, card-mounted sockets eliminates nine connections per 12A_7 and up to six connections for each output tube. Suppose, then that the choice is to use PCB-mounting for the tubes.

In our vision of the amp, we know the general placement of the controls and tubes and jacks. In our modern time, the instinct would be to mount all of these items on one PCB to eliminate as much wiring as possible. Indeed, for a preamp-only or a power-amp-only this is easy to do and does not necessarily incur a weighty service penalty. For a complete amplifier it may be more problematic, so we might have to split the single large board into smaller pieces to make servicing easier.

Splitting the PCB into smaller parts also has the advantage of allowing a modular approach to the development of a product line - a "family" of amplifiers - and/or for easy construction of custom amps. It also allows one to use the less-expensive versions of Eagle or other PCB design software, where the board size is restricted.

Once we make these decisions regarding the board size and what is on it, we should first make a hand drawing with the dimensions for spacing of the externally visible and accessible components, then we can begin drawing the schematic. Because we have an idea of where some things should be placed on the PCB, we can choose to lay out the entire schematic THEN lay out the board, OR lay out a small part of the schematic and start the board, then add a few parts at a time, arranging them as we go along.

Using the entire schematic entry approach is the conventional design intent. You draw the schematic and then select the BOARD icon. Eagle asks "Create new board (named as your schematic)?". Press OK and the board editor opens with a rectangular board outline to the right and all the components arranged to the left. The parts have fly-wires between them showing what lead terminal ties to what other component terminal. It can be quite a mess and intimidating if the circuit is complex. The shapes of the components will make it obvious what each component type is, and they will be laid out in numerical order identical to the order they were dropped onto the schematic. This last detail explains why you might see R1, R2, etc at the middle or end of a schematic rather than at the beginning - the designer or draftsperson simply dropped R1 and used it in the first part of the circuit he/she was thinking about.

Note that (in older Eagle versions) the default board outline is on a 0.05" (50-mil) grid and represents the centre of the milling tool rather than the true board edge. Use the REMOVE tool to get rid of these lines, then use the DRAW tool to lay out a board outline closer to what it should really be. More on these specific actions later.

You should print out the schematic to assist with laying the board. In the board editor, you can use the MOVE command to move parts from the field onto the actual board. If you made that hand drawing of the key dimensions previously, now is when it comes in handy. You can draw reference lines on the board area to show how to precisely align and position the pots, jacks and tube sockets. Move these components first as their positions influence the positioning of all the small resistors and capacitors, and as to how to lay the traces between components.

Fortunately the MOVE feature allows you to enter the geographic designator or NAME of the component into the command line while the cursor is over the board area close to where the next part should be. Enter the name and that part is suddenly stuck to the cursor as a virtual component, ready to be dropped onto the board. left-click to drop the part, then enter the name of the next part to position. Use the ZOOM tools to better see the part of the board you are working on.

Alternatively, since you know the basic lay of the land for the board itself and likely have a schematic in mind, you might do the layout a bit at a time as you build up the schematic. Again, having the hand drawing with key dimensions for placement of the externally accessible parts is essential, although you can also make those decisions within Eagle. Suppose you open the schematic and drop the jacks and pots. Then open the board and make the proper outline for the board and add the reference lines for where the pots and jacks should go. Now move those parts from the field at left onto the board space, lining them up as they should be.

Here is where you can take advantage of the GRID command. Suppose all the pots and jacks are on a 1.5" spacing. Set the grid to 1.5" and for the grid lines to be visible. Now when you move a given component, it can only move in 1.5" increments and everything will be easily lined up to the required spacing. After these parts are in place, reset the GRID back to 0.1" or whatever small increment is appropriate. The dimensions here can just as easily be metric, with "easy" spacings based on millimetre or centimetre multiples. Using this technique might let you save the step of drawing the reference lines, but those are handy to have for other steps later, like laying out a chassis drawing.

Now you have the pots and jacks on the board where they should be and on the schematic in relative positions that may or may not promote a good schematic layout. Move them as required to make space for the next few components. Say we drop the tubes in next. 

Depending on how we have constructed our dual-triode symbol, we are either dropping individual triode sections OR dropping a dual-triode with each ADD command. Using the individually requested triode sections allows for an easier to follow schematic that does not have the signal doubling back on itself - the signal flows from left-to-right as it should. When we add the first triode section to the schematic, the entire dual-triode package is dropped into the unconnected component field in the board editor. The next triode section we drop onto the schematic does not change the board editor. The third triode section we drop adds a new duo-triode package to the board. Using the "component" dual-triode, the heater will be a third component that we drop onto the schematic later using the INVOKE tool. It takes two clicks to drop the two triodes onto the schematic, and a later step to connect the heaters, with the potential to forget about the heater wiring, but also for making a neater schematic.

If we have made the duo-triode symbol as exactly that - a dual triode with six connections - then each request for a dual-triode drops the two-section symbol on the schematic and the package onto the board. Most schematics using this symbol approach also have the heater connections within the symbol, so in fact there are nine-connection points made to the single schematic symbol. Where this is a more realistic presentation of the PHYSICAL component, it is NOT really what we want to have on the schematic. The schematic is an abstract representation of the circuit and should make following the signal and power flow EASIER than it is in the real world construction. Having this all-in-one symbol is intuitive but obfuscating at the same time. It allows each 12A_7 to be dropped onto the schematic and board in one click.
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#2
Incremental vs Complete Schematic capture (continued)

In older versions of Eagle, once you have started to lay out the board any new components added to the schematic will not appear in the field to the left of the board; rather, they will form a vertical line extending down from the origin (0,0). This is a bug and it is possible for this line to get too long to see the last components dropped even with the zoom to fit invoked. In this case, you have to use the MOVE tool and enter the last few part names into the command line one at a time. Otherwise, you can use the GROUP tool to move all the components up to a more easily accessed location, then refit the view of the board.

In general, when using the incremental approach to the schematic and board lay out, it is best to limit each new group of parts added to less than ten or twelve, say. When you see how Eagle spaces the unconnected components, it is clear that these are spaced far enough apart for each to be a 16-pin DIP package (dual-inline integrated circuit outline), which is in keeping for what Eagle was originally designed to do. This manes the field or line of unconnected parts is much larger than you expect it be.

You will find that the incremental and entire-entry approaches are both useful, and you will use them for different projects depending on how complex each new project is. Even when you think the project is complete, it may occur to you that there is space to add a part or two here or there, or some pads for extra wiring for versatile use of the board in the future.
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