# What is Vcc-2 referring to in the Texas Instruments Tech Sheet for the SN74AS138?

Total noob here.

The tech sheet can be found here: http://www.ti.com/lit/ds/symlink/sn74as138.pdf

In the VOH parameter for the SN74AS138 on page 5, it states that it's MIN is VCC-2.

What does that mean? What is referring to? Is it referring to the actual VCC minus 2 volts? So 2.5V for VCC = 4.5V?

Here's the snippet I'm talking about:

It is a simple sum.

It refers to the output voltage when an output is HIGH ($V_{OH}$) as a minimum (it's in the MIN column) of 2 volts below $V_{CC}$, so if $V_{CC} = 5V$ then $V_{OH}$ will be at least 3V.

For CMOS you will often see something like $0.6V_{CC}$, where the value is 60% of the $V_{CC}$ value.

It refers to the pin diagram on the first page, where Vcc is pin 16.

On page 4 of the datasheet, you see:

So it's the supply voltage of the IC.

Also see this: http://en.wikipedia.org/wiki/IC_power-supply_pin - excerpt:

Typical supply pin labeling:

VCC / VDD / V+ / VS+ : Positive supply voltage
VEE / VSS / V / VS− : Negative supply voltage

And exactly, VCC - 2 refers to 'the supply voltage minus two volts'.