Hello Rando,
The USB-6501 does support tristate mode, and the example program you
are using shows the correct method for setting a line to
tristate. When you set a line to tristate, it is not being driven
high or low, but instead to a high-impedance floating state. When
the output is floating, it is putting no load on the circuit, and it is
impossible to determine what the state of the line will be when
measured. It can be driven low or high depending on the device
used to measure the state of the line. What tristate is useful
for is if you have two sources driving the same signal, but only want
one to drive it at a time. Take a look at the following tutorial
for some background on tristate behavior.
Introduction to Wired-OR Outputs and Open-Collector Circuits
Hopefully this helps!
Travis G.
Applications Engineering
