When purchasing a sensor, there are many options. Once a sensor is chosen that fits mechanically and is the general type, there are other considerations to consider during the selection process:
• PNP versus NPN: This is a required option for all solid-state devices. It describes the direction of current flow. PNP is typical in the U.S., but if there’s equipment from other origins, it’s important to know what the PLC input is expecting. If the PLC manual says “sinking input,” it’s PNP; if it says “sourcing input,” it’s NPN. Some input modules can be configured as either. In that case, look at what’s connected to the “common” terminal. If the common terminal is 0 V dc, it’s PNP; 24 V dc is NPN.
• 2-wire versus 3-wire: This is mostly a choice between a mechanical contact (2-wire) and a solid-state contact (3-wire).
• Quick disconnect versus integrated cable: Many sensors offer the option to have a permanently connected cable or a quick disconnect. For a slightly higher cost, the quick disconnect option usually makes maintenance a lot easier. If the sensor breaks, a new cable isn’t necessary.
There was a time when discrete sensors were truly digital in nature such as a mechanical pressure switch using a spring-loaded diaphragm, or a mercury-based thermostat, but the line is blurring. Modern discrete sensors often measure things such as pressure, temperature, inductance, and brightness in analog and convert to a digital yes or no using a tiny computer. Remarkably, many simple sensors now can pass that analog information back to the PLC using technologies like IO-Link (IO-Link Consortium). If the data exists, and there’s a computer in there already, why not take advantage of it? This is a relatively new trend and hasn’t yet found a strong foothold in the market. The PLC and ladder logic programming language were founded on the concept of discrete signals.