hi all, just noticing for long time ago, that 74 series has its open collector companion, and one with max 30V. i dont understand the second, which is not labeled how much the max volts, but it said open collector output, not totem pole one. when/why i need to use this type?

thanks
Posted on 2003-05-08 06:10:11 by dion
Open collector outputs are just that, the output line is the collector of the output transistor. There are two ways these can be used:

1) As a driver to a bus that is connected (common) to other circuit outputs. The transistor can be turned off, thus, not loading the bus down. Then you can connect many of these outputs on the same line without degrading the signal. Frequently it is used on backplanes for plug in cards.

2) If the output is 30v or so, you can have ttl chip have a 30v output because the last transistor is connected externally from the chip to that voltage source. Frequently used to drive analog circuits or transistors.
Posted on 2003-05-08 10:38:40 by drhowarddrfine
Another reason is to tie asynchronous (unsynchronized) outputs to a common active low input, such as an interrupt pin.

Let's say we have an active low interrupt input. That means when it's high, there are no interrupts being requested. With a standard (totem-pole) output, the high state is an active or driven state: there is an output transistor that drives or actively "pulls" the voltage to the high state.

Now you want to request an interrupt. You turn on a transistor to drive the interrupt line low. If there is another transistor driving the line high, the two transistors will "fight" (contend) with each other, and you might not be able to pull the line low enough to cause an interrupt.

By eliminating all of the high-pulling transistors, and replacing them all with a single resistor (the passive pull-up), each low-pulling transistor is free to pull the voltage all the way down to its limit. If all of the transistors are off, the resistor will cause the voltage to rise to (or "float to") the supply voltage value.

Using open-collector (and open-drain for CMOS) is much easier than using or building a many-input OR gate.
Posted on 2003-05-08 16:32:30 by tenkey
Originally posted by tenkey
By eliminating all of the high-pulling transistors, and replacing them all with a single resistor (the passive pull-up), each low-pulling transistor is free to pull the voltage all the way down to its limit. If all of the transistors are off, the resistor will cause the voltage to rise to (or "float to") the supply voltage value.


Remember that the voltage "floats" to the supply voltage because there is not any current draw throught the pull up resistor. CMOS technology if electromagnetic field based technology, meaning no current. If you load this data bus up with a bunch of TTL based chips you might find your "supply level voltage" is more like 4V instead of 5V (due to current draw through this resistor).
Posted on 2003-05-08 19:00:35 by NaN
NaN is correct here, TTL inputs draw current. They sometimes supply current, but that's a story for another day.

So for an open collector summing point as was mentioned, don't expect more then a logic 1 level. But don't be surprised if its greater.

Another handy application for open collector TTL is using it to close a small relay. A 7406 will withstand 30V, and switch 30mA to ground, thats near a watt to play with. I've found some 15V relays that can be directly driven off this output.
Posted on 2003-05-08 19:26:43 by Ernie
thanks for the lesson :grin:
Posted on 2003-05-12 06:17:44 by dion