ExpressSCH TUTORIAL
ExpressSCH is part of a two-program package called ExpressPCB. ExpressSCH is a schematic-capture program used to draw circuit wiring diagrams; The other part, also called ExpressPCB, is a Printed-Circuit-Board (PCB) program used to lay out circuit components on a printed circuit board. Since the latter function is not part of what you do in EE-246, this tutorial focuses only on ExpressSCH.
You can download ExpressPCB from the net for free. Installation should put two icons on the screen. The one for ExpressSCH is shown at the right; ignore or delete the one for ExpressPCB.
When you open the program, you will see the drawing area with grid markings. When you place objects on the screen, they will snap to the grid. (This feature and grid spacing can be changed, but there will probably not be a need for that.)
At the top, there are menu items, and below them, a row of buttons, including zoom in and out. To find the components you'll want to place in the drawing area, right click anywhere in the area and select Component Manager (actually "Component and symbol Manager", referred to below as CSM). In the Library Components window, you'll find the gates and chips you'll be using listed right after the connectors. Skip the 74ALS entries and select from the 74LS series starting with 74LS00. (Note: the letters between 74 and the number at the end represent the electrical, not the logical, properties of the device. Thus, designations like 7400, 74LS00, 74HC00 all describe chips with four 2-input NAND gates. Their logic is the same even though they are somewhat different electrically.)
If you left-click on, say, 74LS00, you will see all four gates displayed on the right. Although the gates all belong to one chip, they are displayed separately (not inside a chip rectangle). The same is true for flipflops and, in general, for any chip containing multiple identical elements. Notice that pin numbers are different for each gate, as they should be. Also, the top gate shows the power and ground pins (14 and 7) for this chip. They serve to remind you not to leave them unconnected.
Now double-click on 74LS00. You will see all four gates in the drawing area. If you don't need all four in your circuit, delete the extra ones from the bottom up. With the mouse, simply draw a box around the extras with the mouse. Hit the Delete key, and they're gone. Then click in the area to turn the remaining ones black. You can enlarge the symbols with the mouse's wheel or by clicking on the zoom button Å at the top of the screen.
A note about the blue color: to activate a symbol, click anywhere on its outline--not inside the symbol--to turn the outline blue. You can now drag it to another place. Or, if you double-click on any part of the outline, you open a window in which you can enter a label for the gate (a part ID).
For example, the gates of the first chip you select should be labeled U1, and each gate should also receive a letter to distinguish it from the others. So the top NAND's ID should be U1A, the next down should be U1B, etc. The next chip you select should be labeled U2. If it also contains multiple identical elements, their IDs should be U2A, U2B, etc.
Suppose you want to build a circuit for the function F = WX + X'Y. You will need 2 AND gates (74LS08), one inverter (74LS04), and an OR gate (74LS32). To find the 74LS32, scroll down the CSM listing past the 74LS0..'s, the 74LS1..'s, and the 74LS2..'s to the beginning of the 74LS3..'s series.
Start by placing all 4 AND gates, 6 inverters, and 4 OR gates in the drawing area. Delete the last 2 ANDs and ORs and the last 5 inverters. The screen should look as shown below at the left.
The inverter has been rotated as shown (click the Rotate 90º button at the top until you get the orientation you want). Also, the gates have been moved into desired positions. Next, the ANDs will be labeled U1A and U1B, the inverter U2A, and the OR will be U3A. Just click twice on the outline of each gate to open the ID assignment window. The result is at the right. (The inverter's text has been dragged a bit to the left so as not to interfere with the wires, which will be added later.)
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Next, circuit inputs and output (W, X, Y, and F) have to be placed on the screen. Right-click and open the CMS listing, but this time click on Library Symbols, not Library Components. For circuit inputs, select "Port-Right pointing-4 letters wide" and double-click. Move the port symbol to the left of AND pin 1. Double-click on the outline and enter the symbol name W. Then right-click on the outline and select Copy. Paste the symbol to the left of pins 2 and 5 and name them X and Y. Return to the Library Symbols list, select "Port-Left pointing--4 letters wide", and place it to the right of OR gate pin 3. Enter F as its name. The result should appear as shown below:
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Lastly, you need to connect all these symbols with wires.
Notice that the arrow button at the top of the list of icons at the left is pressed. This is like the Esc key on a keyboard. To add wires, you have to press the wire button (5th one down), but when you're through, you need to press the arrow again to "escape" from the add-wire mode.
First, run a wire from W's port across to pin 1 (just click and drag across. At pin 1, left-click, then right-click to release the mouse). Repeat from X's port to pin 2 and Y's port to pin 5.
Next, run a wire from pin 1 of the inverter up to the wire from X. Then add a wire from pin 2 of the inverter straight down, click to make a corner, and run it across to pin 4 of the lower AND. Finish up with wires from the two ANDs to the OR. Each wire requires two corners. Finally, connect the OR's pin3 to the output port, F. The result is shown below.
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The equation at the top is optional. It is added by clicking the text button A--the last one down on the left. A window opens at the top of the screen where the text can be entered. Text size can be changed from default value (not done in this case). Then just click on the screen to place the text.
Now that the diagram is complete, you will first want to save it as a .sch document so you will have it if you want to make changes later. Then paste it as a picture into a Word document for your report. Go to the Edit menu and select "Copy schematic image to clipboard". Then open a Word doc and paste. What you will see is the entire drawing sheet, complete with the block at the lower right with Company Name, etc.. The circuit itself will be too small, so you will need to edit the picture in Word.
Click on the picture, and with the Picture Toolbar, select the Text Wrapping tool and choose Square. That will allow you to move the picture about the page. Use the Crop tool to eliminate unwanted sections of the picture, i.e everything except the circuit itself. Then, stretch the picture across the page so that the circuit symbols are large and clear. Since you will print this out for inclusion in your report, you may want to change the color to Grayscale so as not to use your printer’s color cartridge.
DeMorgan (DM) equivalent NAND and NOR symbols
Three files containing these symbols will be made available in class. They are:
IC - 74LS00 - NAND gate - DM.s
IC - 74LS02 - NOR gate - DM.s
IC - 74LS10 - 3-iniput NAND gate - DM.s
These are all .s files, an extension reserved for custom-made files. They do not come with the standard component library; instead, you must store them separately in a special folder that was created when you installed ExpressPCB on your PC. The folder location is:
My Documents\ExpressPCB\SchComponents_Custom.
Now, when you want to insert one of these symbols while drawing a circuit, open Component Manager. Then, click on Custom Components and choose the gates you want. The symbols represent the complement of the usual algebraic definition of NAND and NOR.
NAND: X = (YZ)' = Y'+Z' (an OR with inverted inputs replaces an AND with inverted output)
NOR: X = (Y+Z)' = Y'Z' (an AND with inverted inputs replaces an OR with inverted output)
On the previous page, the circuit for F = WX + X'Y was built with three chips (AND, NOT, and OR). Using NANDs and/or NORs can sometimes reduce the number of chips and, therefore, the physical size and cost of the circuit. The following diagram illustrates this--only one chip is required, a quad NAND 74LS00.
The other advantage of using DeMorgan symbols is that they help preserve the appearance of the original AND/OR circuit and thus makes it easy to follow the logic. In the above circuit, the eye sees inverter bubbles at each end of the wires into the final gate and can ignore them since they cancel each other out.
Suppose we used the standard NAND symbol for the output gate, U1D. The inputs to that gate are (WX)' and (X'Y)', so that F is [(WX)'·(X'Y)']'. This is the same as F = (WX) + (X'Y) but the logic is much harder to follow.
Finally, consider the function F = AB + BC + D'.
For an AND/OR circuit, you would need two 2-input ANDs, an inverter, and a 3-input OR. Since there are no 3-input OR chips, you would have to use two 2-input ORs, for a total of 5 gates from 3 chips.
Now, replace these gates with three 3-input NANDs (74LS10) from only one chip. One of the NANDs has inputs AAB (same as AB), another has inputs BBC (same as BC), and the last, substituting for the two ORs, has inputs from the other NANDs plus D.
Again, this is more compact than an AND/OR circuit, but by using the DM equivalent symbol for the last gate, it preserves the AND/OR appearance, which makes the logic so much easier to follow.
