Electronic – TTL, Totem Pole vs. Open Collector Output
digital-logiclogic-leveltransistorsttl
What is the difference of these two ttl nand gates?
What is a totem pole, what does it do and what is it used for?
When connecting multiple ttl logic gates together is it better to use one or the other?
Best Answer
Totem Pole output drives the output high and low. Open collector will only pull the output low; it does not drive the output high, only releases the output to float.
Multiple open collector outputs may be tied together to operate in a wire-OR arrangement, where any output may pull the output low. In this case, a pullup resistor is used to establish a logic 1 when all outputs are off.
Open collector may also be used to drive non-gate loads, such as LEDs, small solenoids, etc., as long as the load voltage and current is within the rating of the open collector device.
The first comprehensive logic series was the TTL series 74xx. This used BJTs (Bipolar Junction Transistors). Later there came variants like the often used 74LSxx, where the "LS" stands for Low-power Schottky TTL. As the name implies these used less power than the rather power-hungry TTL, and were faster too. At the same time the CMOS 4000 series was developed. The "C" in CMOS stands for Complementary, meaning it's a combination of N-channel and P-channel MOSFETs. Their construction is simpler than TTL and they use far less power. Later standard CMOS developed into HCMOS, "H" for High-speed. Most 74LSxx types have been released as HCMOS in the 74HCxx series, or the 74HCTxx series, which is TTL compatible. Later more variants were developed, like Advanced CMOS (74ACxx).
Microcontrollers are built in HCMOS technology, so they use MOSFETs. AFAIK JFETs aren't used for logic ICs. The transistor you show in the picture is a BJT, which you can tell from the pin designation:
E = Emitter
B = Base
C = Collector
For a MOSFET the pins would be
S = Source
G = Gate
D = Drain
respectively.
Many ICs in the 74HCxx series were originally released in 14 or 16 pin DIL packages, which meant that they would fit four 2-input gates. With miniaturization (SMT) came the demand for smaller packages, even if they contained less gates. Several manufacturers offer single-gate and dual-gate versions of logic gates. For example, NXP has a 74LVC1G00 (single 2-input NAND) and a 74LVC2G00 (dual 2-input NAND) version of the classical 74HC00. 74LVCxx is yet another HCMOS technology. This page lists all NXP logic families.
Do you know that the horizontal strips of a breadboard are all connected together (except sometimes in the middle)? To me this looks like you have shorted the input to the output of the 7805. (Or maybe the output to te ground, my view is blocked by the capacitor.)
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OK, next step. You are using a plain old 7400, no LS or HC or any other letters between the 74 and 00? That old chip requires 5V. If you used it at 9V you can't assume it is still working correct. It might, but don't count on it.
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I can't make sense of your LED question. You don't mention a series resistor, did you use one?A LED nearly always requires a series resistor. For starters, take 1k.
Best Answer
Totem Pole output drives the output high and low. Open collector will only pull the output low; it does not drive the output high, only releases the output to float.
Multiple open collector outputs may be tied together to operate in a wire-OR arrangement, where any output may pull the output low. In this case, a pullup resistor is used to establish a logic 1 when all outputs are off.
Open collector may also be used to drive non-gate loads, such as LEDs, small solenoids, etc., as long as the load voltage and current is within the rating of the open collector device.