EGR 278 Digital Logic Lab File: N278L3A Lab # 3 Open-Collector and Driver Gates

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1 EGR 278 Digital Logic Lab File: N278L3A Lab # 3 Open-Collector and Driver Gates A. Objectives The objectives of this laboratory are to investigate: the operation of open-collector gates, including the use of pull-up resistors and wire- ANDing. the use of drivers and output transistors to provide high current levels for loads B. Materials Breadboard Tektronix 5280 DC ower Supply Agilent 33401A Digital Multimeter 5V ower Supply Micronta Digital Logic robe Fluke Digital Multimeter 12V Automobile Marker Bulb (#194) One 7400 Quad 2-input NAND IC One 7401 Quad 2-input open-collector NAND IC One 7406 Hex Inverter Buffer/Driver (open collector) 7400, 7401, 7406 Specification Sheets Any N transistor with I C (max) >1A (such as SK3441, NTE 292, TI30C, or TI42 ). C. Introduction TTL logic circuits typically have one of two types of outputs: Totem-pole outputs Open-collector outputs Totem-pole outputs This is the most common type of output for TTL devices. Figure 1a below shows the internal circuitry of a input NAND. The transistors in TTL devices (denoted by Q) essentially act like switches. When a transistor is ON it acts like a closed switch and when a transistor is it acts like an open switch. Note that transistor Q 4 is stacked on top of Q 3 like a totem-pole. The output Y is taken from the top of Q 3. Figure 1b shows the current path through the totem-pole when the output is and also when it is LOW. When the output is : Q 4 is ON, Q 3 if, and the current I OH flows through Q 4 and out Y. Note that I OH = 0.4 ma maximum. When the output is LOW: Q 4 is, Q 3 if ON, and the current I OL flows in Y and through Q 3 to ground. Note that I OL = 1.6 ma maximum. Figure 1A NAND circuit Figure 1B - current paths

2 age 2 Open-collector outputs TTL circuits with open-collector outputs have only the lower transistor (Q 3 ) seen in the previous totem-pole output. Since there is no internal path from the output Y to the supply voltage, the circuit does not function properly unless an external pull-up resistor is used.. Figure 2a below shows the internal circuitry of a 7401 open-collector 2-input NAND. Figure 2b shows a 7401 NAND with an external pull-up resistor attached. Y (output) Figure 2a open-collector NAND Figure 2b NAND with pull-up resistor Note that a special symbol is shown inside the NAND gate shown above. Open-collector gates are not always denoted with special symbols, but they may be indicated using one of the four symbols shown in Figure 3 below. * OC Figure 3 - Symbols for open-collector gates Why should we use open-collector gates which require the addition of a pull-up resistor in order to function properly when we could use a gate with a totem-pole output instead? There are several reasons: 1) Wired-ANDing - Open-collector outputs can be tied directly together which results in the logical ANDing of the outputs. Thus the equivalent of an AND gate can be formed by simply connecting the outputs. This is especially convenient when large numbers of signals need to be ANDed. 2) Increased current levels - Standard TTL gates with totem-pole outputs can only provide a current output of 0.4 ma and a LOW current of 1.6 ma. Many open-collector gates have increased current ratings. 3) Different voltage levels - A wide variety of output voltages can be achieved using open-collector gates. This is useful in interfacing different logic families that have different voltage and current level requirements.

3 age 3 Wired-ANDing If TTL gates with totem-pole outputs have their outputs tied together, the gates may be destroyed.. This is illustrated below in Figure 5 where the top gate has a output and lower gate has a LOW output. 130 I Q4 ON D3 Q3 Y (output of gate #1) (outputs of the 2 gates connected) 130 I > 16 ma (resulting in destruction) Q4 D3 LOW Y (output of gate #2) Q3 ON Figure 4 - The destruction of totem-pole gates if their outputs are tied together When open-collector gates have their outputs tied together the result is that the outputs are ANDed together. This is illustrated with two examples in Figure 5 below. I I LOW LOW LOW = LOW = Figure 5 - Examples of wired-anding using open-collector gates

4 age 4 ull-up resistor calculation ull-up resistors are typically in the range of several hundred ohms to several thousand ohms, but exact ranges can be calculated using the relationships shown in Figures 6 and 7 below. Case 1: All input gates (ANDed result is ) Used to calculate R (max) I Calculation for (max): V CC - VOH (min) R (max) = M(I ) + N(I ) OH IH I OH I OH I OH implied wired-and operation I IH I IH I IH Example: Using M = 4 and N = 3 as shown on the left and using standard TTL specifications: R (max) = 4(250µ A) + 3(40µ A ) R (max) = 2321 Ω I OH N = 3 loads M = 4 input gates Figure 6 - Calculating the maximum pull-up resistor value Case 2: At least one input gates is ON (ANDed result is LOW) Used to calculate R (min) I Calculation for (min): V CC - VOL(max) R (min) = I (max) - N(I ) OL IL ON I OL implied wired-and operation I IL I IL I IL N = 3 loads Example: Using M = 4 and N = 3 as shown on the left and using standard TTL specifications: R (min) = 16mA - 3(1.6mA ) R (min) = 410 Ω M = 4 input gates Figure 7 - Calculating the minimum pull-up resistor value

5 age 5 Open-collector drivers Some open-collector gates are particularly well suited to driving loads that require higher voltage and current levels, such as incandescent lamps and relays. An example is the 7406 Hex Inverter Buffer/Driver. This gate is similar to the 7404 Hex Inverter (totem-pole output) except that I OL (max) = 40 ma (instead of 16 ma) and V OH (max) = 30 V (instead of around 5V). An example is shown in Figure 8 below where the output of a 7406 is used to drive an incandescent lamp that requires a 12 V supply and about 35 ma of current. Note that a currentlimiting resistor R limit may or may not be necessary, depending on the resistance of the lamp filament. 12 V R limit bulb Figure driving an incandescent lamp The 7406 allows us to drive loads that require up to 40 ma of current. For even larger currents, it may be necessary to use an output transistor. Transistors are available that can provide currents of several amperes. An example is shown in Figure 9 12 V B C E bulb Figure 9 - Using an output transistor to provide output currents up to several amperes

6 age 6 D. reliminary Work 1. Calculate minimum and maximum values for pull-up resistors under the following conditions. Use specifications for the 7401 (assume that V OH (min) = 2.4V). Tabulate your results. Show a sample calculation for one case. M (# inputs) N (# outputs) resent a documented circuit layout only for Circuit 4B in the Laboratory Work section. 3. Generate a truth table for the circuit shown in Figure 10 below where the outputs of the open-collector gates are wired-anded together. In other words, simply generate the truth table for F = A B C. A A B B F = A B C C C Figure 10

7 age 7 E. Laboratory Work 1. Open-collector gate without a pull-up resistor Connect a 7401 open-collector NAND with its inputs tied together (to act as an inverter) and with no load and no pull-up resistor as shown in Circuit 1. Measure the output with a voltmeter for both and LOW inputs. Are the output voltages within specified ranges? (They should not be!) A Y (output) Circuit 1 Input Switch A Output Voltage Specified Voltage Within specs? L H 2. Open-collector gate with a pull-up resistor Connect a 7401 open-collector NAND with its inputs tied together (to act as an inverter) and with no load using a pull-up resistor as shown in Circuit 2. Use a pull-up resistor that is within the range calculated in Section D. Record the value of resistance used. Measure the output with a voltmeter for both and LOW inputs. Are the output voltages within specified ranges? (They should be!) V cc R p A Y (output) Circuit 2 Input Switch A Output Voltage Specified Voltage Within specs? L H

8 age 8 3. Using a wired-and configuration with totem-pole outputs Connect Circuit 3 using a 7400 NAND for only 2 or 3 seconds (or the gates may be destroyed) and record the output voltage and current. Is the current within specified limits for a LOW output? Ammeter LOW Voltmeter Circuit 3 4. Using a wired-and configuration with open-collector outputs A) Connect Circuit 4A using a 7401 open-collector NAND using a pull-up resistor that is within the range calculated in Section D. Record the value of resistance used. Record the output voltage and current. Are the current and voltage within specified limits for a LOW output? Why is the output LOW and not? Ammeter LOW Voltmeter Circuit 4A

9 age 9 B) Connect Circuit 4B using a pull-up resistor that is within the range calculated in Section C. Record the value of resistance used. Note that the gates on the left are 7401 open-collector NANDs and the gates on the right are 7400 NANDs. Verify the truth table determined in Section D (i.e., determine the logic level of the output for all possible input switch combinations add an LED and a current-limiting resistor to show when the output is or LOW). Are the outputs of the open-collector gates ANDed together as predicted? Switch A Switch B Switch C Circuit 4B 5. Driving an incandescent lamp using an output transistor Connect Circuit 5. For which switch position does the lamp light? Measure and record the current delivered to the lamp for both input switch positions as well as the current and voltage at the output of the Compare the current measured to the lamp when it is lit to the max specified current for both the 7401 and the Was the transistor necessary? Also compare the output voltage of the 7406 with standard TTL voltage levels. 12 V Switch '06 I o 2.2k B E SK3341 N BJT C TI 30 bulb #194 12V auto marker light I bulb B C E Circuit 4 TI 30 in Assignment Circuit 5

10 age 10 F. Report 1. resent all measured data from the Laboratory Work clearly and concisely. Discuss the results. 2. Use tables whenever possible to compare measured values to specified values. 3. Discuss any values that do not fall within specified limits and suggest reasons for the discrepancies. 4. Compare gates with totem-pole outputs and gates with open-collector outputs. How are they different? 5. Discuss the issue of output current. Compare the output current levels (particularly I OL (max) ) for a 7400, 7406, and an output driver transistor.