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Unformatted text preview: Massachusetts Institute of Technology Department of Electrical Engineering and Computer Science 6.111- Introductory Digital Systems Laboratory How to Make Your 6.111 Project Work There are a few tricks of the trade which allow an experienced digital designer to fix problems quickly. This note is an attempt to fill you in on some with which you might not be familiar. Wiring Errors 1. Do you have a diagram of your circuit? Not that piece of napkin from commons last night, I mean a DIAGRAM. If you dont, then think about making one. Making the diagram may in itself lead you to finding the problem. A timing diagram is often helpful for finding errors as well. 2. Now that you have a diagram, it should be easy to have your partner check the wiring. Not you. Your partner. You will make the same mistake that you made in wiring your circuit a second time when you check it. 3. Check each IC package. Is there a wire on each of its pins? There probably should be, in most cases. Explain the cases where there is not (unused section of 74LS00, parallel inputs not used on counter . . . ). You would be surprised at how many errors are caught with this simple technique. Be careful especially on the power and ground signals. 4. The pin assignments of a 74LS02 are not the same as those of a 74LS00. When you use them, double check the pin numbers. Care and Feeding of the Power Supply 1. Dont overload the kit power supply. If in doubt, measure the power supply with an OSCILLOSCOPE, not a VOM. Many of the power supply overload problems show themselves as hum or ripple on a supply rather than low voltage output. 2. Grid your power supply distribution. This means construct an X-Y pattern of power interconnections for the ground and power. The extra wires are redundant if you be- lieve you are working with superconducting zero inductance wires. You are not. Recall that KCL requires that signals which force current to ow OUT a wire also cause the SAME CURRENT to ow BACK on the ground and power wires. 3. This means that if you are, for example, building a project with multiple kits, you should be quite careful to provide enough grounds between the kits. Id say a con- servative minimum is about 1 ground wire for each 3 signal wires. The ground wires should connect diverse points on the grid of each kit, not all be connected to the power 6.111 How to Make Your 6.111 Project Work 2 supply pin. Dont connect the +5 volt power on adjacent kits together. You can check the adequacy of your grounding between kits by measuring the ground potential dif- ference between points in your project with a scope. Worry about anything more than about 0.5 volts....
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