The inputs and outputs are all provided by a single device, a Xilinx CPLD, XC9536. The digital outputs provide TTL-compatible levels for driving logic devices. But you can also control devices, such as relays and light emitting diodes, that require more current. You can either provide current from a digital output high, or you can sink current into a digital output low. Generally speaking, the output low is capable of sinking much more current than the output high can source, and using it results in lower power dissipation. So if you need substantial current, you should turn on the external device by sinking current from it using an output low.
As an output is loaded, its voltage level, VOL or VOH, rises or falls with the current. It is often useful to know just how much to expect the VOL and VOH levels to degrade with current. For currents of less than 100 mA the voltage change is approximately linear with current; that is, it can be modeled as a voltage source of either zero volts (for an output low) or 3.3 volts (for an output high) and an equivalent series resistance. At greater output currents the resistance becomes nonlinear, but you shouldn�t ever source or sink more than 100 mA.
The following figure shows the degradation of output voltage levels with increasing source or sink current for a typical device.
Consulting the VOL vs I curve for the output pin we find that at 10 mA VOL = 0.12V. We therefore need a resistance near 270 ohms.