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Some basic gpio/ATTiny help needed

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First, please forgive me, I'm a Software Engineer by trade and very new to any actual electronics so it's quite possible I'm asking a very stupid question, but I've been stuck on this for a few hours so figured I'd ask the experts.

I'm using a ATTiny85 to send a signal to a gpio board (Devantech USB-I2C, but just using the pins as io not I2C) and can't seem to get it to work correctly. The USB-I2C board has an internal pullup resistor such that after setting the pin on the USB-I2C board to input the pin reads 5V. When I connect my ATTiny85, no matter if I send a signal on the pin connected to the USB-I2C or not, the USB-I2C pin always reads high.....

If anyone could provide me some insight on how this is supposed to work I would greatly appreciate it.
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