I need to connect digital output pin of Arduino Uno to Keithley DAQ6510 digital multimeter external trigger input. Both are utilizing 5V TTL logic.

Now when I measure the voltage on external trigger input of DMM, I see 5V. So I assume there is some pull up resistor involved just like they have it for digital input pins. My question is whether I can connect this input to Arduino Uno's digital output pin or this input is assuming dry contact control using relay (connect input pin to the ground for initiating trigger). Connecting 5V to 5V seems little unsafe to me so I would like to confirm.

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  • \$\begingroup\$ Why do you think connecting 5V to 5V is unsafe? \$\endgroup\$
    – JYelton
    Commented Jun 3, 2022 at 16:40
  • \$\begingroup\$ @JYelton maybe because this will create undefined current flow. My knowledge in this area is small, that's why I ask. I am coming from software world. \$\endgroup\$
    – Pablo
    Commented Jun 3, 2022 at 16:57
  • \$\begingroup\$ The image you added pertains to the DB9 I/O connector of the DMM, not the external trigger input (which is a BNC). Just so we're on the same page. \$\endgroup\$
    – JYelton
    Commented Jun 3, 2022 at 20:50

1 Answer 1


It's true that if you connect 5V to a microcontroller GPIO pin, without any limitations to current, and then configure that pin as an output pin and pull it to ground, a lot of current will flow and exceed the MCU specs (and the magic smoke will be released).

But the multimeter trigger input is not the same as a power supply, and its 5V is a high-impedance TTL implementation:

Keithley DAQ6510 EXT Trigger In Figure

(Ref. Manual, pg 64)

In other words, it's not providing 5V with an amount of current that could damage the microcontroller. The Arduino Uno working voltage is also 5V, but be aware that other microcontrollers and even some Arduinos use 3.3V in which case you would use a buffer or level shifter.

  • \$\begingroup\$ Thanks a lot! But what exactly implies to high-impedance implementation? I've updated my post with screenshot of digital I/O port configuration from the same documentation. As I have the same "issue" on digital I/O ports (measuring 5V on input), then I would like to understand if 2x 100k resistors are ensuring high impedance implementation or FET? \$\endgroup\$
    – Pablo
    Commented Jun 3, 2022 at 19:37
  • \$\begingroup\$ @Pablo The interfacing considerations section of the TTL Wikipedia article sort of explains it. Basically the transistors are like a buffer wherein you don't have current flowing aside from the small amount necessary to operate the transistors themselves. You might want to search for or ask as a separate question when to use current-limiting resistors. \$\endgroup\$
    – JYelton
    Commented Jun 3, 2022 at 20:49

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