USB-8451 DIO line loss of control

[Scott Youngren]

I am using the USB-8451 [OEM mounted on my custom PCB.] I am using a CVI 8.1 application.

The SPI functions for this application are working nicely. 

Two of the DIO lines, 0 & 1 connect to a ULN2003A to drive 2 relays. [The remaining lines are inputs -working fine]

The voltage type is the default 3.3V push-pull.

Most of the time,  setting these 2 lines work fine. Occasionally, the lines fail to respond.

Unplugging and reconnecting the USB connection, restarting the application to INIT the driver

usually restores the DIO output line function.

I have even tried another USB-8451 card.

I would appreciate any suggestions for a reliable performance.

Scott Youngren

[ColeTrain]

Hey Scott,

As you're using these lines to drive relays, this can often have a large load on the digital lines.  If the line cannot recover to the 3.3 V state, it will become unresponsive and will need to be reinitilized (much like what you are seeing).  You can decrease your odds of seeing this by adding an external resistor to the 5V line on your 8451.  This shouldn't be less than 1 kΩ.  Try this out and let us know if you're still seeing this behavior.

[Scott Youngren]

ColeTrain

Thanks for your help.

While I and my partner continue to research this problem, I attach a simplified drawing [pdf] of the circuit to remove any doubt of communications between us!

I tried the open-collector mode, but this does not work until software initializes this mode. I am reverting back to the 3.3v push-pull mode as the circuit 'pulls down' the default input mode until the S/W defines D0/D1 as outputs.

I am carefully reviewing the S/W app. [& driver] start-up and shut-down processes. It appears that "closing" the USB driver leaves the 8451 DIO lines in the input/output state defined.

Then re-starting the S/W & driver redefines the setup. Only powering down/powering up the USB-8451 puts the device into its "default" DIO lines as input state.

Is this correct? I will work other issues until your kind response!

Thanks

Scott

[AnalogKid2DigitalMan]

Where is your ground connection to pin 8?

-AK2DM

[Scott Youngren]

Cole

Sorry, the Orcad capture object has the pin 8 ground hidden, but appears on the net list. It is there!

Scott

[Scott Youngren]

more on the missing ground on the schematic...One of my other fellow engineers hates the 'hidden pin' feature of Orcad for exactly your comment! He turns them on=visible on his schematics.

Scott