Digital Logic Fundamentals Tri-State Output Exercise 2: Source and Sink Current EXERCISE OBJECTIVE When you have completed this exercise, you will be able to demonstrate how a tri-state buffer output can source and sink current by using the TRI-STATE OUTPUT circuit block. You will verify your results with an oscilloscope and by observing the output LEDs. EXERCISE DISCUSSION to the negative supply (ground). ction from ground (negative) to the positive supply. Transistor-transistor logic (TTL) circuits, such as a tri-state buffer, function as current devices. For the output of a tri-state buffer (TTL device) to source or sink current, the device has to be in the high or low output state, not in the high-z state. An output of a tri-state buffer is in the high-z state. The buffer can a. source current to an output load. b. sink current from an output load. c. not source or sink current. FACET by Lab-Volt 249
Tri-State Output Digital Logic Fundamentals Current from a TTL device is source current: the device supplies current to an output load. Current from an output load to the TTL device is sink current: the device provides a current path from the load to ground. The output of a tri-state buffer is high (logic 1). The buffer can a. source current to an output load. b. sink current from an output load. c. not source or sink current. When the tri-state buffer input is high, the output is high (logic 1) because the top transistor (Q1) of the totem pole is turned on. CC (5 Vdc) through the Q1 collector-emitter, the resistor, and the red LED to ground. LED); the buffer is the source for the current to turn on the red light. The output of a tri-state buffer is low (logic 0). The buffer can a. source current to an output load. b. sink current from an output load. c. not source or sink current. 250 FACET by Lab-Volt
Digital Logic Fundamentals Tri-State Output When the tri-state buffer input is low, its output is low (logic 0) because the bottom transistor (Q2) of the totem pole is turned on. CC (5 Vdc) through the resistor, the green LED, and the Q2 collector-emitter to ground. the buffer sinks the current that turns on the green LED to ground. When both transistors in the totem pole output of a tri-state buffer are not conducting, the buffer can a. source current to an output load. b. sink current from an output load. c. not source or sink current. When the tri-state buffer is disabled, both transistors (Q1 and Q2) are turned off, and the output is in the high-z state. FACET by Lab-Volt 251
Tri-State Output Digital Logic Fundamentals In the high-z state, the tri-state buffer cannot source current to a load or sink current from a load because the buffer is effectively disconnected from the load. The bottom transistor in the totem pole output of a tri-state buffer is conducting current from an output load. The buffer is a. a current source for the output load. b. a current sink for the output load. c. in the high-z output state. PROCEDURE Locate the TRI-STATE OUTPUT and INPUT SIGNALS circuit blocks. Put a two-post connector in the terminals at BLOCK SELECT. 252 FACET by Lab-Volt
Digital Logic Fundamentals Tri-State Output Connect A at the INPUT SIGNALS circuit block to A (INPUT) at the TRI-STATE OUTPUT circuit block. Connect B at the INPUT SIGNALS circuit block to B (OUTPUT ENABLE) at the TRI-STATE OUTPUT circuit block. At the INPUT SIGNALS circuit block, set toggle switches A and B in the HIGH position. This puts a logic 1 (high) at the data input (INPUT) and at the enable input (OUTPUT ENABLE) of the tri-state buffer. With highs at INPUT and OUTPUT ENABLE, what is the logic state of the output? a. logic 0 b. logic 1 c. high-z FACET by Lab-Volt 253
Tri-State Output Digital Logic Fundamentals Connect the channel 1 probe of the oscilloscope to the OUTPUT of the tri-state buffer. Connect the probe ground clip to a ground terminal on the circuit board. tri-state buffer output is logic 1? a. yes b. no With highs at INPUT and OUTPUT ENABLE, is the tri-state buffer a source or a sink for the output current? a. source b. sink a. top b. bottom Is this schematic a valid representation of how the red LED connects to the tri-state buffer? a. no b. yes 254 FACET by Lab-Volt
Digital Logic Fundamentals Tri-State Output At the INPUT SIGNALS circuit block, set toggle switch A to the LOW position. This puts logic 0 (low) at the data input (INPUT). With a low at INPUT and a high at OUTPUT ENABLE, what is the logic state of the output? a. logic 0 b. logic 1 c. high-z tri-state buffer output is logic 0? a. yes b. no FACET by Lab-Volt 255
Tri-State Output Digital Logic Fundamentals With a low at INPUT and a high OUTPUT ENABLE, is the tri-state buffer a source or a sink for the output current? a. source b. sink a. top b. bottom Is this schematic a valid representation of how the green LED connects to the tri-state buffer? a. no b. yes At the INPUT SIGNALS circuit block, set toggle switch B to the LOW position. This puts a logic 0 (low) at the enable input (OUTPUT ENABLE) and at the data input (INPUT). 256 FACET by Lab-Volt
Digital Logic Fundamentals Tri-State Output With lows at INPUT and OUTPUT ENABLE, what is the logic state of the output? a. logic 0 b. logic 1 c. high-z With lows at INPUT and OUTPUT ENABLE, is the tri-state buffer a source or a sink for the output current? a. source b. sink conducting? a. top b. bottom CONCLUSION load. When a tri-state buffer output is in the high-z state, it does not source or sink current; there is no REVIEW QUESTIONS 1. A TTL device, such as a tri-state buffer, functions as a a. voltage device. b. capacitive device. c. current device. d. impedance device. FACET by Lab-Volt 257
Tri-State Output Digital Logic Fundamentals 2. A tri-state buffer can a. source current. b. sink current. c. have a high impedance output. d. All of the above. 3. This tri-state buffer is a. a current source. b. a current sink. c. in the high-z output state. d. not conducting. 4. This tri-state buffer is a. a current source. b. a current sink. c. in the high-z output state. d. not conducting. 5. When this tri-state buffer is sinking current, what transistor in the totem pole output circuit is conducting? a. top b. bottom c. top and bottom d. neither 258 FACET by Lab-Volt