Digital Ac Input Module

HI
REfer to attached file can somebody explain what is negative true logic thanks

The AC input is isolated using an opto isolator. The opto-isolator is a photo transistor and an LED. Your AC power flows throught the LED, causing it to emit photons, which are picked up by the NPN transistor, turning it on. The NPN transistor then clamps the DC potential at a pull up resistor to ground, making it go low. Thus, when AC power is on at the input the internal DC voltage that represents the state of the input is low. When AC power is off at the input the internal DC voltage that represents the state of the inputis high. This is a function of the hardware, not a function of the software logic. Software logic is still 1=true, 0=false.

I have trouble with Alaric's analysis. Maybe I misread it. When ac current is flowing the NPN photo-transistor is indeed turned on by the output of the LED. This will cause current to flow through the base resistor and the NP junction of the output transistor causing it to conduct. The collector of the output transistor will then go low as it is conducting.

The voltage at 'OUTPUT' will be low with respect to 'LOGIC GND' when ac current is flowing. It will be high with respect to 'LOGIC GND' when ac current is not flowing. How that is interpreted by following circuits is purely a function of the design of those circuits.

As many expect a high voltage when a logical condition is TRUE then the circuit may be said to provide negative (or backwards from what is expected) logic. As long as you are aware of the voltage states you can call it anything you want to.

Both descriptions are accurate.
The module you show is for 5 volt logic. In positive logic, a True is 5 volts, False is 0 volts. In 'Negative True Logic', it's inverted. True is 0 volts, False is 5 volts.
They just used a fancy term to say 'Output is Inverted'.

Dear Alaric,

Knowladge of screwdriver level ,who can't handle program.A screwdriver carry is essential for a programmer.

Rgds,
tarok.seal

Beware of programmers who carry screwdrivers.--Leonard Brandwein

TRUE .
I back Alaric

It indicates they intend to do more than drink, Coke or Coffee

tarok.seal said:

Dear Alaric,

Knowladge of screwdriver level ,who can't handle program.A screwdriver carry is essential for a programmer.

Rgds,
tarok.seal

Click to expand...

I see you grasp one of the two essential meanings of Mr. Leonard Brandwein's statement. The statement is a double entendre, that is it can have two interpretations, either one of which could be correct with respect to any individual programmer.



Bernie,
We were both saying the same thing - but I think you phrased it better.

Alaric - looking at your first post for the 10th time I realise I did misread it the first 9 times. Sorry about the implication.

As to the screwdriver - while I think my co-workers respect my programming and troubleshooting ability, if I dare to actually pick up a screwdriver or wrench they'll approach carefully saying 'Bernie, put down the tool and step away from the machine.' Maybe they're trying to say something.