Several different resistors are used on Digilent boards. Some are used to limit LED current, and some are used on inputs (like the button and switch circuits) to both limit the currents flowing to the main chip, and to help protect against electrostatic discharge (or ESD – more on this topic later). The resistors on the Digilent boards, like most resistors used in digital systems, are physically small because they will not encounter large voltages or currents. For these smaller resistors, the resistor value in Ohms is printed in microscopic numbers on the resistor body, not visible to the naked eye.
In a PCB that uses through-hole technology, holes are drilled through the pads so that component leads can be inserted and then fastened (soldered) in place. In a PCB that uses surface-mount technology, component leads are soldered directly to the pads on the surface. Each set of pads (or holes) in the PCB is intended to receive a particular component. To identify which component must be loaded where, reference designators are printed on the circuit board immediately adjacent to the pads using a silk-screen process. A parts list links a designated set of pads to a physical component by describing the component and assigning it a particular reference designator. The reference designators guide assemblers and testers when they are working with the PCB. Many components must be placed into the PCB in a particular orientation. By convention, components that require a particular orientation have one lead designated as pin 1. On the PCB, a square pad or silkscreen indicator typically denotes pin 1.
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