Active Low and Active High Signal?

[MarcSylex]

Not really a question but I felt there's no other way to bring up a topic this topic.

As we all know an active low signal means that something is asserted only when an incoming/outgoing signal is pulled low from a high value.

The reverse is true for active high, it is only asserted when the incoming/outgoing signal is high.

My question/open discussion is whether as engineer I should look at the behavior of the signal and the affect it has on an io port to determine the active state or should I (we) look at the port itself. I might sound like I'm mincing words but please hear me out.

It would rather sound better if a particular port is designated as an active low port, which attachs a behavior on something physically liable for that actually determining a systems reaction. Let me descrbe a simple example:

12V with some series resistance in series with the collector of BJT. The output (which goes to a port) of BJT goes to anode of an LED with cathode in series of resistance which goes to GND. BJT emitter goes ot GND. Assume some method for switching BJT ON/OFF.

Lets say our goal is to turn ON the LED.

With this it would be more accurate to describe the output port as an active HIGH port since the setup is what determines the port's "signal's" behavior.




I guess to me signals don't care what they are High or Low but what does matter is how the port or system recieving that signal reacts. Will it react to a low or high signal? Is the port active high or active port.

 

[itsmoked]

If your port needs a high signal to, say, turn on a bit or instead turn on a lamp then it's active high. Doesn't matter if ultimately a 'device' goes low to do it.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MikeHalloran]

For a simple two-wire circuit there are actually six states of interest:

sender hi|lo

receiver hi|lo

sender unterminated

receiver unterminated

I've come across a fair number of system designs where a 'driver' board, with high power devices and its own power source, is controlled by a separate 'controller' board, and the driver board _turns_everything_on_ when the cable between the two is intentionally disconnected, or falls off.

Engineers check for and prevent things like that.



Mike Halloran
Pembroke Pines, FL, USA

 

[benta]

In logic design, it is a good idea to decouple the logic function from the actual voltage level when designing and analysing circuits.
In your example, you have a signal we can call "LED_ON". The name implies what this signal does. The actual voltage level of asserting this signal is only an impementation question.
When you make the actual circuit, you rename the signal to "LED_ON(H)" (could also be "LED_ON(L)", of course).
In this way it's completely clear what the signal does, and what the assertion voltage/level is.

Regards,

Benta.

 

[IRstuff]

Your choice should be dictated by what makes the most SNR sense. In most cases, one particular leg is the noisiest and requires negative true to maximize the noise immunity.

Then again, there's always differential, so in those cases, the question is moot

TTFN

FAQ731-376

 

[VEBill]

Often the design choice is driven by careful consideration of common failure modes.

For example, you might not want a important indicator (such as 'Landing Gear Down And Locked') to be fooled by common wiring failures (such as an open circuit, or a short circuit to ground). Thus, such an indicator might be designed to be active high.

On the other hand, 'Door Ajar' on your car can be active low because the failure modes are not serious.

 

[melone]

_b or _n are common ways to indicate active low signal polarity. In other words, LED_ON is an active high signal, while LED_ON_b or LED_ON_n is an active low signal.