A second problem with tri-state busses can happen when two outputs are fighting"one another over the bus. Large currents and overheating of the design can occur under these circumstances and the logic levels on the bus will be middle" Thermal debugging of designs is a quite efficient test strategy, since it is very quick and can find some problems with very little work. Check to see that all of your parts are dissipating some power. Are any so hot that you cant touch them? Unless you are using some part with an excuse to be hot, then some output is probably trying to pull down the +5 volt power supply, or fighting with some other part Open Collector Signals Calculate the resistor value carefully. To do this, calculate the total input current of all the loads on the run. Subtract this from the output drive available from the driver part. If more than one type of part is being used, use the lowest output drive. The remaining current is available to pull down the resistor. The resistor value is then given by R=5/ohms This will usually be in the several hundred ohm range PNPInputs Newer TTL parts, those with numbers above 300 for the most part, use a low DC current input circuit using a PNP transistor. These parts have a very low TTL current, which might make you think that you can drive hundreds of them with a single part. This strategy is dangerous if you value the time and signal quality of your design. AC considerations mean that you should limit the number of signals on a run to the 10-15 range regardless of the DC current specification. The same comment applies, with even more force, to the MOS inputs of static memory parts and EPROMS. These parts have high input capacitance which makes driving them with a reasonable number of loads essential Handling cmos Parts CMOS devices are static sensitive. They have sufficiently high input impedance that the static charge stored on your body capacitance can permanently damage them. Ground yourself to the bench and your project before touching these devices, and they will work when you are finished plugging them in Wire routing Wires, as we mentioned above in the section on power supplies, are not perfect components. They have a parasitic inductance, resistance, and a mutual inductance with other wires. There are some strategies for minimizing the effect of these imperfections in our ideal wire modelA second problem with tri-state busses can happen when two outputs are ``fighting" one another over the bus. Large currents and overheating of the design can occur under these circumstances, and the logic levels on the bus will be ``middle". Thermal debugging of designs is a quite efficient test strategy, since it is very quick and can find some problems with very little work. Check to see that all of your parts are dissipating some power. Are any so hot that you can't touch them? Unless you are using some part with an excuse to be hot, then some output is probably trying to pull down the +5 volt power supply, or fighting with some other part. Open Collector Signals Calculate the resistor value carefully. To do this, calculate the total input current of all the loads on the run. Subtract this from the output drive available from the driver part. If more than one type of part is being used, use the lowest output drive. The remaining current is available to pull down the resistor. The resistor value is then given by: R = 5/I ohms This will usually be in the several hundred ohm range. ``PNP" Inputs Newer TTL parts, those with numbers above 300 for the most part, use a low DC current input circuit using a PNP transistor. These parts have a very low TTL current, which might make you think that you can drive hundreds of them with a single part. This strategy is dangerous if you value the time and signal quality of your design. AC considerations mean that you should limit the number of signals on a run to the 10-15 range regardless of the DC current specification. The same comment applies, with even more force, to the MOS inputs of static memory parts and EPROMS. These parts have high input capacitance which makes driving them with a reasonable number of loads essential. Handling CMOS Parts CMOS devices are static sensitive. They have sufficiently high input impedance that the static charge stored on your body capacitance can permanently damage them. Ground yourself to the bench and your project before touching these devices, and they will work when you are finished plugging them in. Wire Routing Wires, as we mentioned above in the section on power supplies, are not perfect components. They have a parasitic inductance, resistance, and a mutual inductance with other wires. There are some strategies for minimizing the effect of these imperfections in our ideal wire model