Digital I/O

cancel
Showing results for 
Search instead for 
Did you mean: 

NI USB 6525 Digital Input Signal Voltage

I have connected my circuit as in the attachment. From the voltage divider, i should have get a voltage reading of around 4.8V(LOGIC HIGH) when my X06 signal supply is 24V DC. However, i can only get a voltage reading of 1.8V(LOGIC LOW) when i measure the voltage at port 1.0+ and 1.0-

Anyone face the same issue as mine? How to solve the problem? Please advise. Thanks.

Message 1 of 4
(4,426 Views)

Hi,

 

If you calculate the voltage across the resistors you are right. But the input resistance of the input is not high enough to be neglected. According the specification the input current is max. 3mA. So the resistance in parallel of the 1k resistor will result in a lower resistance and lower voltage.

 

Why do you use the voltage divider ? 

 

Kees

0 Kudos
Message 2 of 4
(4,400 Views)

As already mentioned, using a voltage divider does not necessarily work with TTL compatible inputs since they have their own specs concerning input resistance (or rather input current in high and low state). 

 

Also, in case one of the resistors (or solder points) fails, there will be +24VDC on the input which might damage the device. 

 

To protect a TTL input from overvoltage, you should use an opto-isolator. You can drive the input LED with the 24V signal and a current limit resistor, and there are opto-isolators (AKA opto-couplers) available with TTL compatible output. 

0 Kudos
Message 3 of 4
(4,291 Views)

Hi xuijing, 

 

This is an interesting question you are having. 

 

As ports P1.0 - 1.7 and PFI ports are input ports, it is expected that the voltage you supply to the voltage divider is less than 4.8V as a part of the voltage is lost to the input port. Instead of using the P1.0+ and P1.0- ports, try wiring your voltage divider to P0.0A and P0.0B ports, as P0.0 - 0.7 ports are output ports. 

 

Also, if you don't mind, may I know why are you wiring a 24V input to the output port P0.0 ?

 

You may get additional info from this link, just download the specifications document to learn more about NI USB 6525: 

https://www.ni.com/en-us/shop/model/usb-6525.html

 

Hope this helps! 

D.S Yiauw
National Instruments | ELP Engineer
CLAD
0 Kudos
Message 4 of 4
(4,272 Views)