Hex inverter and open collector

[mvoltin]

I am just learning electronics and have difficulty differentiating between the hex inverter and open collector. Here is the specific question:

1. Hex inverter, such as 74HC04: In digital logic, an inverter or NOT gate is a logic gate which implements logical negation. Thus, when input is low, the output is high and the other way around.
2. Open collector such as 74LS06 is basically a hex inverter with a pull-up resistor.

So, is it accurate to say that the main difference is that the open collector, instead of just being "LOW", actually "pulls" the output circuit to ground when low (basically, "connects" whatever is on the other side of the output to ground)? I know my semantics are terrible (I am new to this) but hope you understand my point. Is this a correct assumption? thanks.

 

[audioguru]

An ordinary logic part has two output transistors. One pulls the output low and the other pulls the output high.
An open collector output is missing the transistor that pulls the output high so that a resistor can be used instead then more than one open collector output can be connected together to make a multi-input NOR gate (if one or more than one input is high then the output is low, if all inputs are low then the output is high).

 

[KeepItSimpleStupid]

TTL logic is typically a totem pole type structure. It's actively driven in both directions.

The open collector inverter has somewhat been depreciated because of devices like the ULN2003. Recently there was a similar device that was introduced that works at lower logic voltages.

Sink current is usually high with an OC inverter, so lamp LED driving was a typical use. You can convert an OC nverter to an inverter with a pull up resistor, but FAN out might be affected.

FAN out is a term used to describe the # of inputs an output can drive.

There is a lot more than meets the eye and some manufactures have a "logic family" description. It makes sense to understand that document first for it defines what is a logic high and what is a logic low. Now, it's usually described as a percentage of Vcc because Vcc can vary. CMOS puts high and low really really close to the supply rails making interfacing logic families hard.

The "pull up" structure is used a lot with switches, so an OC inverter can effectively create an OR to a switch input. Multiple OC devices can be placed in parallel to get an effective OR gate.

You have to get your head around, that a high on the input of an OC gate causes a low on the output and might turn on a LED or activate a switch.

One BIG advantage of the ULN2003 type devices is interfacing, It will tolerate an input that is open. An open state is OFF. A logic one is ON. On the other hand, if an OC inverter is used for the same interfacing, if you disconnect the (interface), the stuff that it's connected to, turns on because of the inputs of the OC inverter float high.

This is also a practical reason why NOT ENABLE is used on motor drivers.

Another advantage of the ULN2003 type devices in microprocessor interfacing. Many ports were defined as inputs at power up and when these are inputs, they won;t affect the state of a relay when the port is changed to an output.

The TI DRV777 https://www.google.com/url?sa=t&rct...wzSwMepntZieU7w&bvm=bv.70138588,d.aWw&cad=rja is a new generation driver similar to the ULN2003.

 

[JimB]

then more than one open collector output can be connected together to make a multi-input NOR gate

A bit more terminology, this kind of connection is sometimes referred to as a "wired NOR".

JimB

 

[ramuna]

mvoltin asked:
So, is it accurate to say that the main difference is that the open collector, instead of just being "LOW", actually "pulls" the output circuit to ground when low (basically, "connects" whatever is on the other side of the output to ground)? I know my semantics are terrible (I am new to this) but hope you understand my point. Is this a correct assumption? thanks.
Answer: Yes!
See: https://www.ti.com/lit/ds/symlink/sn74ls06.pdf (Section:Schematic, each gate)
For the output to become low, the rightmost npn transistor (open collector) switches ON and pulls whatever voltage source it's open collector is connected to, to ground potential.

 

[dr pepper]

Sounds like yoy pretty much get it.
TTL such as 74hc (actually cmos 'ttl'), stands for transistor transistor logic, the first two words mean 'transistor' pull to ground, and 'transistor' pull to +.
Early logic now obsolete used RTL, this had a pullup resistor in the chip and only one transistor - resistor transistor logic.
Open collector (or open drain if a fet) is handy for wire or -ing, a common application is a network bus in a vehicle, beacuse th eline is pulled up even if more than one device tries to takecontrol of th ebus a short circuit isnt possible like it would with ttl.

 

[eTech]

mvoltin asked:
So, is it accurate to say that the main difference is that the open collector, instead of just being "LOW", actually "pulls" the output circuit to ground when low (basically, "connects" whatever is on the other side of the output to ground)? I know my semantics are terrible (I am new to this) but hope you understand my point. Is this a correct assumption? thanks.
Answer: Yes!
See: https://www.ti.com/lit/ds/symlink/sn74ls06.pdf (Section:Schematic, each gate)
For the output to become low, the rightmost npn transistor (open collector) switches ON and pulls whatever voltage source it's open collector is connected to, to ground potential.

Hi

Also...(referring to LS06)...because the collector is not internally connected to VCC, it is "open", therefore, it can be connected to a different voltage source than what would be provided if it were internally connected to VCC. However, the cost of this convenience is that an external "pull-up" resistor or load must be used to connect the "open" collector to its voltage source.

eT

 

[MrAl]

Hi,

There are actually two types of open collector gates for TTL and related logic families.

The first is just open collector with a voltage rating equal to the supply, usually 5 volts.
The second is also an open collector, but the internal transistor can handle a much higher voltage than the supply voltage of 5 volts.

They are both usually used as some sort of interface logic. The second could be used as a level shifter for example, while the first could be used as part of a delay circuit that discharges a capacitor and allows that capacitor to charge through an external resistor, where the open collector is used so that the output of the gate can not charge the capacitor it can only discharge it.
Another use of course is a 'wired' logic gate, where we need a lot of inputs but just one output and cant get this feature in a standard gate package. In this case we can connect many gates outputs together and that can give us a lot of inputs.