【正文】 er scan. In this case the address for ’first scans’ in a PLC5 is ’S2:1/14’. With the logic in the example the first scan will seal on ’light’, until ’clear’ is turned on. So the light will turn on after the PLC has been turned on, but it will turn off and stay off after ’clear’ is turned on. The ’first scan’ bit is also referred to at the ’first pass’ bit. Figure 6 A programs that checks for the first scan of the PLC 7. MEMORY TYPES There are a few basic types of puter memory that are in use today. 可編程邏輯控制器（ PLC ） 21 RAM (Random Access Memory) this memory is fast, but it will lose its contents when power is lost, this is known as volatile memory. Every PLC uses this memory for the central CPU when running the PLC. ROM (Read Only Memory) this memory is permanent and cannot be erased. It is often used for storing the operating system for the PLC. EPROM (Erasable Programmable Read Only Memory) this is memory that can be programmed to behave like ROM, but it can be erased with ultraviolet light and reprogrammed. EEPROM (Electronically Erasable Programmable Read Only Memory) – This memory can store programs like ROM. It can be programmed and erased using a voltage, so it is being more popular than EPROMs. All PLC use RAM for the CPU and ROM to store the basic operating system for the PLC. When the power is on the contents of the RAM will be kept, but the issue is what happens when power to the memory is lost. Originally PLC vendors used RAM with a battery so that the memory contents would not be lost if the power was lost. This method is still in use, but is losing favor. EPROMs have also been a popular choice for programming PLC. The EPROM is programmed out of the PLC, and then placed in the PLC. When the PLC is turned on the ladder logic program on the EPROM is loaded into the PLC and run. This method can be very reliable, but the erasing and programming technique can be time consuming. EEPROM memories are a permanent part of the PLC, and programs can be stored in them like EPROM. Memory costs continue to drop, and newer types (such as flash memory) are being available, and these changes will continue to impact PLC. 8. SOFTWARE BASED PLC The dropping cost of personal puters is increasing their use in control, including the replacement of PLC. Software is installed that allows the personal puter to solve ladder logic, read inputs from sensors and update outputs to actuators. These are important to mention here because they don’t obey the previous timing model. For example, if the puter is running a game it may slow or halt the puter. This issue and others are currently being investigated and good solutions should be expected soon. 9. SUMMARY 可編程邏輯控制器（ PLC ） 22 ? A PLC and puter are similar with inputs, outputs, memory, etc. ? The PLC continuously goes through a cycle including a sanity check, input scan, logic scan, and output scan. ? While the logic is being scanned, changes in the inputs are not detected, and the outputs are not updated. ? PLC use RAM, and sometime EPROMs are used for permanent programs. 10. PRACTICE PROBLEMS ? Does a PLC normally contain RAM, ROM, EPROM and/or batteries? ? What are the indicator lights on a PLC used for? ? A PLC can only go through the ladder logic a few times per second. Why? ? What will happen if the scan time for a PLC is greater than the time for an input pulse? Why? ? What is the difference between a PLC and a desktop puter? ? Why does PLC do a self check every scan? ? Will the test time for a PLC be long pared to the time required for a simple program? ? What is wrong with the following ladder logic? What will happen if it is used? ? What is the address for a memory location that indicates when a PLC has just been turned on? 11. PRACTICE PROBLEM SOLUTIONS ? Every PLC contains RAM and ROM, but they may also contain EPROM or batteries. ? Diagnostic and maintenance ? Even if the program was empty the PLC would still need to scan inputs and outputs, and do a self check. ? The pulse may be missed if it occurs between the input scans 可編程邏輯控制器（ PLC ） 23 ? Some key differences include inputs, outputs, and uses. A PLC has been designed for the factory floor, so it does not have inputs such as keyboards and mice (although some newer types can). They also do not have outputs such as a screen or sound. Instead they have inputs and outputs for voltages and current. The PLC runs user designed programs for specialized tasks, whereas on a personal puter it is unmon for a user to program their system. ? This helps detect faulty hardware or software. The self check helps detect these types of faults, and shut the system down safely. ? Yes, the self check is equivalent to about 1ms in many PLC, but a single program instruction is about 1 micro second. ? The normal output Y is repeated twice. In this example the value of Y would always match B, and the earlier rung with A would have no effect on Y. ? S2:1/14 for micro logy, S2:1/15 for PLC5. 12. PLC technique discussion and future development Along with the development of the ages, the technique that is nowadays is also gradually perfect, the petition plays more strong。t satisfied with the current manufacturing industry foreground, also can39。s labor strength, reduce the establishment on the personnel. The target control of the hard realization in many plicated production lines, whole and excellent turn, the best decision etc., welltrained operation work, technical personnel or expert, governor but can judge and operate easily, can acquire the satisfied result. The research target of the artificial intelligence makes use of the calculator exactly to carry out, imitate these intelligences behavior, moderating the work through person39。s machine bine, for resolve the very plicated problem to lo