plc英文翻译

plc英文翻译内容摘要：

s possible. But in a program elements can only be examines one at a time in a fixed sequence. Consider the ladder logic in Figure 4, the ladder logic will be interpreted lefttoright, toptobottom. In the figure the ladder logic scan begins at the top rung. At the end of the rung it interprets the top output first, then the output branched below it. On the second rung it solves branches, before moving along the ladder logic rung. 毕业设计（论文）外文翻译 ７ Figure 4 Ladder Logic Execution Sequence The logic scan sequence bee important when solving ladder logic programs which use outputs as inputs. It also bees important when considering output usage. Consider Figure 5, the first line of ladder logic will examine input A and set output X to have the same value. The second line will examine input B and set the output X to have the opposite value. So the value of X was only equal to A until the second line of ladder logic was scanned. Recall that during the logic scan the outputs are only changed in memory, the actual outputs are only updated when the ladder logic scan is plete. Therefore the output scan would update the real outputs based upon the second line of ladder logic, and the first line of ladder logic would be ineffective. Figure 5 A Duplicated Output Error 4。 PLC STATUS 毕业设计（论文）外文翻译 ８ The lack of keyboard, and other inputoutput devices is very noticeable on a PLC. On the front of the PLC there are normally limited status lights. Common lights indicate。 power on this will be on whenever the PLC has power program running this will often indicate if a program is running, or if no program is running fault this will indicate when the PLC has experienced a major hardware or software problem These lights are normally used for debugging. Limited buttons will also be provided for PLC hardware. The most mon will be a run/program switch that will be switched to program when maintenance is being conducted, and back to run when in production. This switch normally requires a key to keep unauthorized personnel from altering the PLC program or stopping execution. A PLC will almost never have an onoff switch or reset button on the front. This needs to be designed into the remainder of the system. The status of the PLC can be detected by ladder logic also. It is mon for programs to check to see if they are being executed for the first time, as shown in Figure 6. The ’first scan’ inp ut will be true on the very first time the ladder logic is scanned, but false on every other scan. In this case the address for ’first scan’ in a PLC 5 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 An program that checks for the first scan of the PLC 5。 MEMORY TYPES 毕业设计（论文）外文翻译 ９ There are a few basic types of puter memory that are in use today. 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 PLCs 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 PLCs. 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 a。