What does an NMOS transistor with the gate connected to the drain do?
Those are depletion-mode1 NMOS transistors. They function as pull-ups or &quot;loads&quot; (effectively resistors) for the wired-NOR NMOS logic. When the source+gate connection is pulled low, they basically become current sources.
The key point is that pull-down (enhancement mode) transistors are placed at the cross-points in both the AND-plane on the left and the OR-plane on the right in order to implement the desired logic functions. The result is a direct implementation of a sum-of-products form of logic.



simulate this circuit &ndash; Schematic created using CircuitLab

1 As indicated by the fat &quot;channel&quot; line; an old shorthand notation. The other transistors in the diagram are the more familiar enhancement-mode devices.