Tri-state with transistor?

You're using 'tri-state' a bit differently than the industry norm. What you're referring to is called 'open-collector'.

True tri-state devices have the following 3 possible output states:

Active-low (logic 0, output pulled to ground - base-high in your example) Open ( output is open circuit, base-low in your example)

Active-high(logic 1, output pulled to +V by an upper transistor in a 'totem pole' arrangement)

Replace 'tri-state' with 'open collector' and you're absolutely right.

Not usually. Most IC outputs can provide several mA of current; proper selection of your transistor and its base resistor will keep you within spec...

Check the datasheet for the IC you have in mind.

Yep. There are also 'open'collector' versions of the common logic gates available.

HTH

So is this to say that "open-collector" is the same as the high-impedance state something like a buffer or latch goes into when its output is disabled? From what I gathered from your post, this is possibly the case, but I'm wanting to make absolute sure. For the signal I have in mind, I'd want to be able to take it from being forced 'low' (aka tied to ground) to being the equivalent of being disconnected (or floating, whatever you want to call it).

Hmm, would something like an open-collector inverter be able to perform the same task I'm referring to? I'd put the toggler line on the input, and the togglee on the output, and this would supposedly pull the togglee low when the input was high, and let the togglee float when the input was low?

Yup that'd work , you've just "invented" the open-collector output.

Another trick is to put a diode in series with the ic's output.

That depends on the IC and the transistor. if it's an issue use two (or more) transistors.

As long as your using small transistors and only switching typical logic level signals it will not be an issue.

Bye. Jasen

LOgic families usually can supply and source a voltage or current. That means they switch between a high level and a low level. "Low" is not an absence of voltage, it is an active positioning of the output to ground or close to it.

You can't connect anything in parallel, because one may try to be high while the other gate output is trying to be low (bad for the ICs, and unless it's really controlled, you don't want the results).

But sometimes you want to be able to select between two outputs connected to the same point, and devices with tri-state outputs are one solution. The in effect disconnect the output of the IC, the point that goes between high and low, from the output pin, so the circuitry connected to that pin will never know something is connected there. It looks like the IC is never actually connected to the circuit. Then you can run the other device connected to that point like nothing else is there.

This is the high impedance state, the "tri-state" that is neither high nor low.

An obvious example is a computer data bus, when many devices need to output to that bus feeding the CPU. Using tri-state devices ensures that only one device is driving the data bus at a time.

Open collector devices can only provide a low, ie ground or close to it. But they require some external voltage source in order to operate, without that voltage there is nothing to pull "low", even if the transistor could operate without any voltage on the collector. ICs that provide open collector devices are usually used to drive external circuitry, where levels will not be at the logic level of the device. Thus you can provide the needed pull up resistor to the +supply, the resistor supplying the "high" to the following circuitry. Or, the open collector device is feeding something that doesn't need a "high" output, but merely the low. These are things that have their other side connectoed to the +supply, such as relays and Nixie tubes, and speakers.

Michael

I seem to have a knack for "inventing" things. Maybe next I'll invent the telephone.

Hey that does sound like an interesting trick indeed. I don't think I actually have any around, though, but I do have some transistors. I'll have to check and see if they're the right kind for the job.

Something I was wondering about is whether I need to put a resistor between the emitter and ground. Or between any of the other transistor connections, for that matter. I've never connected an IC's output to ground before, and while I think there might be resistance in the transistor to prevent a short, I'm not going to go wiring it up and take a chance on burning everything out just yet!

At risk of sounding like an even bigger beginner, I'm afraid I can't make heads or tales enough out of the datasheets for the transistor type I have and for the IC I want to activate it with.

My toggler line to the transistor base is coming from a 74LS374 (octal flip-flop), and the transistors I have are PN2222A. First, I found the datasheets..

..but as mentioned, I don't understand how to read the charts well enough to figure out what volts/amps are output from the one and required for input on the other. I was looking in the "On Characteristics" of the transistor datasheet, but it says stuff like

50ma at the base and 500 at the collector, then 2v to the right of that. I don't get what that means, exactly, if that's even what I'm supposed to be looking at. I looked at the electrical characteristics for the '374 as well, but I can't find an output current.

Something I did notice about the '374 is that if I'm understanding the sheet, its active signals are actually around 3.4v, instead of the 5v I assumed. I've never really looked at the electrical characteristics for IC's before, and while I know 3.4v is enough to be TTL 'high', maybe this is just common voltage output for most of'em.

I also want to say thanks to everyone who's replied so far, it's nice to know there's folks who know their stuff out there willing to help!

No. A tri-state device can put the output to +V, ground or open.

Our open-collector (transistor) idea has only two possible outputs: ground or open.

Here's a drawing of the output stage of a logic gate, buffer or latch; the 'totem-pole' I mentioned:

Vcc | |/ -| |>

| +---- output | |/ -| |>

| | === GND (created by AACircuit v1.28 beta 10/06/04

If the top transistor is turned on, the output is connected to Vcc (logic 1). If the bottom transistor is turned on, the output connects to ground (logic 0). Think of the open-collector circuit as the bottom half of a tri-state circuit.

If *both* transistors are off, you get the 'open' state.

If both transistors are ON, you get the 'smoke' state. ;)

See also:

==================== Here's an interesting application for an open-collector gate: As long as the max. voltage and current are low enough, a 5v logic gate can control a higher (or lower) voltage load.

Be sure you read the datasheet on the gate before you try this, though!

1.5v-18v | 5v +-+ | | | load |\\ +-+ -| >O--| |/ | === GND

(created by AACircuit v1.28 beta 10/06/04

Below is basically what you need. The 2.7K (to 10K) resistor limits the current fed into the base of the transistor to an acceptable level. e.g. When the transistor is forward biased, (turned on), the base will be at about 0.6V. If the chip's output is 3.4V, then the voltage across the resistor will be 3.4-0.6V=2.8V. 2.8V into a 2.7K resistance will be about

1mA. Anything from about 2.7K to 10K should do the job fine.

... Fred

5V | | +------ 0V / Hi-Z +------+ | | | |/ | '374 |---[2.7K to 10K]---| PN2222A | | |> +------+ | | | === === = = GND GND

Be sure to use a resistor in series with the base of your open collector transistor.

Rb = [ V(high) - 0.6v ] / ( Ic/beta )

...jerry