Difference between logic voltage and open collector?

[xcpointx]

Can anyone tell me IF there is a difference between
logic voltages/signals
and open collector outputs?

If there IS a difference how can I determine which is which using a multimeter if possible.

[Peter Snowberg]

All standard logic uses push-pull output stages to it will actively output a voltage level. Open collector outputs are more like "pull only" and don't have the "push" transistor connected from the output to power so they are only able to output a logic 0, and they don't output anything when standard logic would be outputting a 1.

This comes in really handy when you want to externally apply a switched ground connection to a circuit, or if you want to create passive "OR" style logic. It also has the bonus of making outputs that don't fight each other if they're outputting opposite states. If you hook two pieces of standard logic together and one is outputting a 1 while the other does a 0, you'll let the blue smoke out of the chip. With OC logic, the 1 is provided by a "pull up" resistor that connects to V+ (10K is pretty common) and logic 0 is provided by an OC output turning on.

One example of OC being used to protect a bus structure is SCSI. As an interface it was a great thing to have come along. The use of open collector +5 volt signaling allows you to plug or unplug SCSI connections at any time without any danger of blowing chips (don't plug or unplug anything from a bus you're using if you want the data to remain intact). SCSI terminators are just active versions of the "pull" resistors. Terminators eat excess signals on the bus but they have a second purpose which is to provide the logic 1. The old terminators used resistor divider voltage sources to try to place a logic 1 voltage of roughly 3 volts on the bus. Newer units use voltage regulators to apply 2.85 volts.

By just adding a pull up resistor, you can use OC logic right along with standard logic.

When you ask about looking for output type with a multimeter that can be tough but it can be done in many cases. I assume this is for some +5 volt TTL logic. The easiest way is to just check the part numbers of the chips involved. If they're rubbed off, you can often spot OC logic devices by the pull-up resistors on the outputs. With a meter you could also look at voltage on the output. The "pull" transistor has a forward voltage drop. Let's use 0.6 volts just to pick a number. This means that a logic 1 is a transistor switched version of the V+ line which in TTL is +5 volts, so your logic 1 voltage might be 4.4 volts. If you see 4.7 volts on your meter then you may be seeing some OC logic in action. The voltage you see will depend on loading. If the output is CMOS, logic 1 will look almost the same as the +5 volt rail thanks to the different switching nature of MOSFET devices. What you see will be greatly influenced by the quality and input resistance of your meter.

I would look for pull up resistors instead.

Take care,
-Peter

[xcpointx]

thank you Peter, that cleared everything up.

I measure changes between 0V and 0.61V on the output.
I can use a pull up resistor between this output and V+ to get switching between 0V and 4.9V (or thereabouts)

I want to get this logic to ultimately switch between -15V and GROUND, but I have no clue as to how to do it!?

Please, maybe you can help me with this:
http://www.diystompboxes.com/sboxforum/viewtopic.php?p=142983