Active Low and Active High Signal?
Active Low and Active High Signal?
(OP)
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.
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.





RE: Active Low and Active High Signal?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Active Low and Active High Signal?
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
RE: Active Low and Active High Signal?
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.
RE: Active Low and Active High Signal?
Then again, there's always differential, so in those cases, the question is moot
TTFN
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RE: Active Low and Active High Signal?
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.
RE: Active Low and Active High Signal?