What's the practical difference between active drive and open collector?

[achb]

I've done some searching and I understand what it is, but I don't understand what it means in practical terms. Why use one or the other?

[SebasAce7]

Can you elaborate on what active drive is? I'm not familiar with the term...

Open Collector Applications: 

Because the pull-up resistor is external and does not need to be connected to the chip supply voltage, a lower or higher voltage than the chip supply voltage can be used instead. Open collector circuits are therefore sometimes used to interface different families of devices that have different operating voltage levels. The open-collector transistor can be rated to withstand a higher voltage than the chip supply voltage. Such devices are commonly used to drive devices such as Nixie tubes, and vacuum fluorescent displays, relays or motors which require higher operating voltages than the usual 5-volt logic supply.

[achb]

Active drive also known as push pull. http://digital.ni.com/public.nsf/allkb/D9B4018C3E540CE4862570F30068C0DB

But I don't really understand in what specific situations you would use either active drive or open collector.

[crossrulz]

Active Drive is what a normal digital output is, typically TTL or LVTTL.  The Digital Output supplies the voltage.  These are typically used when trying to drive digital circuits (which is what I most often run into).

SabasACE7 gave you a good list of uses for the Open Collector outputs.

[achb]

So you would not use open drain for TTL?

What if I want to use my DAQ's digital output to drive a pin low (by default it is 5V)? I know for an active high pin it is straight forward, since to drive it high you just connect it to my DAQ's 3.3V supply. But for an active high pin, that means the pin is already at 5, supplied by the device itself...then I'm not sure what to do exactly. In that case to pull the pin low I want my DAQ to provide a path to ground, to sink current. Does active drive/open collector configurations have anything to do with this? Sorry, this is still new to me, so often I'm not even sure what the right question to ask is.

[Buechsenschuetz]

Actually active drive (or TTL compatible) outputs will provide a path to ground when in low state. For bipolar TTL circuits (like the 74LS series of IC) this is a must since the input of the follow-up device will recognize a low state only when there is a path to ground. 

Of course open-collector outputs will also provide a path to ground when in low state. The main difference is that on open collector outputs you will see no output voltage when in high state. The output voltage must be provided by the load, i.e. usually the load is connected between Vcc (usually 5.0 or 3.3VDC) and the output, and the load (a solenoid for example) will be activated when the output is in low state. 

You could also use the voltage available on the output of an active drive output. But this voltage is not guaranteed to be at the full Vcc level (for TTL compatible outputs the minimum value is about 2.8VDC) and also current capability is limited by the output transistor. So in most cases it is not a good choice to drive a load with an active drive output. These are rather designed to interface with other logic circuits. 

[Minions]

Here is an easy concept: 

You want to drive a 24Vdc relay coil of 600 ohms in order to connect standard 110/220Vac for operating a lamp.  However, they do not have 24V logic.  Having the relay tied to a 24Vdc source on the excitation(+) end, you can use an open-collector circuit connected to the return line (-) of the relay that is capable of driving over 40mA.  (Conceptual only)

The relay will be off (high-impedance state on the relay return) until the open-collector circuit is driven to ground which will then power the relay circuit (24Vdc to Ground across the relay coil) making the connection and thereby enabling power to turn on the lamp.

[Minions]

Visual attached for the Open-Collector concept.  The active drive would require using a 5VDC relay (matching the uP/CPLD voltages) and use the gate input to drive the relay ON or OFF.