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can draw your programs as ladders, or write them as series of instruction codes, which is the method I prefer. And instead of using a computer, you can also write and edit instruction code from a handheld keypad, but these only show a few lines of code, so it’s hard to keep track of what you’re writing. Programmed as instruction codes, our flip-flop relay becomes: LD X1 OR M1 ANI X2 OUT M1 The LD (load) command starts a new rung on the ladder, OR and ANI mean logical OR and ANDInverted, and OUT outputs to coil M1. Converting equipment from relay logic to ladder logic running on a PLC makes it simpler. One of my older coin-op machines, The Doctor (Figure G), writes and dispenses an illegible prescription form. I made the original version in 1987 using relays, and its correct operation depended on a lot of micro switches triggering everything in the right order. A few years ago I replaced the relays with a PLC and was amazed how much less cluttered it became. The accurate timers in the PLC made half the original micro switches redundant. The machine is now much simpler, more reliable, and even has a xenon beacon that flashes when the prescription forms run out.

PLC HISTORY: WIDE TRACES AND RUBBER HAMMERS Modicon introduced the PLCs to replace cabinets full of relays with a digital equivalent. They developed a new architecture so processors would be fast enough to simulate ladders of fast-switching relays within a high-level processing language. To make PLCs more reliable than the computers of the time, they increased signal-to-noise ratio by making everything bigger: larger ferrite cores for memory and wider tracks on the circuit board. They also avoided fans; outside air could carry dirt and cause corrosion. Modicon engineers built a test chamber called “the blue box.” A PLC had to run for a minimum of 24 hours in arctic and tropical conditions. It was then vibration tested, run next to a Tesla coil to test electromagnetic interference, and hit repeatedly with a rubber hammer. PLCs soon grew more sophisticated, supporting mathematical functions and flowchart programming to run subsections of the ladder. Real-time clocks were added, but never widely used because they were irrelevant to most industrial control. They represented the year with just 2 digits, which I suspect inspired some of the “millennium bug” panic. Several years ago, soft PLCs came out. These turned Windows PCs into ladder-programmable PLC-alikes, but they never caught on. Small, limited operating systems are more reliable than anything running on top of a complete PC operating system. Make:


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