Wiring LED to Driverstation

[sircedric4]

Section 3.1.6.2 Driver Station Digital Outputs of the 2009 FRC Control System manual says: "The DS digital output pins are driven directly by 5V transceiver (SN74LVC4245A device). Be sure to use caution as each pin can source/sink more than 40mA. It is possible to damage the device if too much current is driven on a single pin or the total current supplied by the device on all pins is excessive. Best practice should be to use a limiting resistor to prevent external loads from drawing excessive current, especially for LEDs, be sure to use a series resistor to limit the current." The overview of Driver Station ports earlier in the manual also mentions them: "Digital Output pins 5V output level (8) – Provides 5V TTL for interfacing to digital devices and to drive LEDs (with current limiting resistors)."

I read this which was why I got a resistor with a current drop like I did. My problem was figuring out which of the 3 wires from the digital output should I put to my 2 wire resistor. I didn't want to mess up my DriverStation by wiring it wrong through trial and error.

That Digi-Key part is already wired, big, and ready for panel mounting so I figured why re-invent the wheel if they had a part I could use? :-)

So to repeat I go from red 5V from digital output through my LED and then to the signal pin on the digital output? Is this correct? And then my program can turn it off and on by cycling the digital output in code?

[Alan Anderson]

Originally Posted by sircedric4

So to repeat I go from red 5V from digital output through my LED and then to the signal pin on the digital output? Is this correct? And then my program can turn it off and on by cycling the digital output in code?

It will work that way, though the LED will be off for a "1" value and on for a "0". Since the DS's output can source current as well as sink it, I'd connect from the digital output through the indicator to ground instead. That way the LED will be off for "0" and on for "1".

The way you connect it should depend on whether you want the LED to be on or off when the system is reset, before the cRIO software has a chance to control it.

[sircedric4]

Originally Posted by Alan Anderson

It will work that way, though the LED will be off for a "1" value and on for a "0". Since the DS's output can source current as well as sink it, I'd connect from the digital output through the indicator to ground instead. That way the LED will be off for "0" and on for "1".

The way you connect it should depend on whether you want the LED to be on or off when the system is reset, before the cRIO software has a chance to control it.

I originally tried it this way, from the center pin (indicator, signal) through to ground in the direction where the signal wire went into the positive side of the LED and it didn't do anything when I commanded it to. I thought I was just wiring it wrong. I may have to check to make sure the PWM cable was hooked to the correct pin. Couldn't FIRST have bothered to label their pins, its so easy to miscount!

Thanks everyone, I imagine with the info here I can get it to work. I'll let you know after I get a chance to try it tonight. I can't exactly bring the control system to work so have to wait till the evening to play with it.

[Mark McLeod]

Originally Posted by sircedric4

I originally tried it this way, from the center pin (indicator, signal) through to ground in...

The top pin is signal.
The center pin is 5v.
The bottom pin is ground.

Wiring from the center pin to ground should leave the LED on constantly.