This section introduces the memory structure and functions of the Siemens S7-200PLC. The memory of the PLC can be understood as a large-scale integrated circuit module with memory function. The location is close to the CPU (Zhu Yuanzhang) for reading and writing programs and data. Its internal is divided into three parts, namely: ROM memory, RAM memory, EEPROM memory. Figure 1
1, ROM memory, its performance characteristics are equivalent to the BIOS of our computer motherboard, storing the PLC hardware input and output driver, data bus, communication bus interface information and other important code, we can not access the data in this part of the area Can not be modified. It is like the blackest black box of the private body of the CPU (Zhu Yuanzhang). There are only things in the bottom that can only be seen by Lao Zhu. If you are not a supernatural person with a stunt, don’t hit it. So we know that there is such a thing for ROM memory, as long as you don't want to crack the PLC, we will not use it for a lifetime.
2, RAM memory, this memory features the equivalent of our computer's memory, the various data that needs to be calculated when the program is running, the speed of reading and writing data is very fast, the disadvantage is that the data can not be saved after the PLC is powered off . When we program later, all the operating objects we need to use are inside. That is to say, the write program is a register for writing various functions in the RAM memory, and they include (I, Q, AI, AQ, M, V, T, C) (S, SM, L). Seeing no, just a few, and beginners just need to understand the 8 registers in the front brackets, the previous understanding of the brackets in the back will understand. So RAM memory is the focus of our learning.
3, EEPROM memory, this is simpler, it is equivalent to the hard disk of our computer, the program we write is stored here, the broken program will not be lost. After we have written the program, connect the PLC, plug in the download line, click (download) and it will be OK. How is the program stored after entering the PLC? How long does it last? How fast is it? Regardless of him, where is the memory, those are the CPU (Zhu Yuanzhang) to do the work.
This EEPROM memory store program does not have to go too far for us, but there is a special area we need to know, and future programming may be used. In addition to storing the program, the EEPROM memory also leaves an area dedicated to the data that needs to be saved for a long time after the PLC is powered off. As we mentioned above, the data in the RAM memory can not be saved after the power is turned off. Unfortunately, when our program is running, the data of the operation is placed in the RAM memory, so in order to permanently save some data in the RAM memory, This area has been specially created in the EEPROM memory. Although there is a super capacitor inside the PLC, it can save data for us after the PLC is powered off, but it can only last for about 3-7 days. The data in the capacitor will still be lost after the power consumption. But with this area, we set it through the programming software, and the PLC will write the data we need to save in the RAM memory to the special area in the EEPROM memory for permanent storage at the moment of power failure. We can think of this area as the place where the CPU (Zhu Yuanzhang) saves money in private houses. The private money is placed in the safest place! Now, as long as we know that there is such an area, we will study it later when we actually use it.
Next we talk about RAM memory, because everything we need to program is in it. As shown in Figure 2, this is the overall structure of the RAM memory. The Siemens s7-200 PLC divides the RAM memory into register areas for many different functions. The memory size varies depending on each function. Figure 2 shows the RAM memory divided by register size in bytes (BYTE). What is byte (BYTE) We have already said in the previous lesson, if you don't understand, let's take a look at the contents of the previous lesson. The three registers in the picture are in the beginning, we don't care about them. The other eight registers are the ones we need to focus on, because when we program, we edit them, which are (I, Q, AI, AQ, M, V, T, C) registers. The (I, Q, AI, AQ, M, V) registers have been introduced in the previous lessons and will not be repeated here. Just talk about the T register and the C register.
T is the timing register. It is the alarm clock, but the alarm clock of PLC is more than the alarm function of getting up from us. The number is more than one. It is divided into three types according to the accuracy: 1 millisecond (VIP level, not much). 10 milliseconds (member level, medium number), 100 milliseconds (the largest number of visitors).
C is the counting register. In fact, it is a count. When you kick it, it counts a number. When you kick it to a certain amount, it clears the number yourself and lets you kick it from the beginning. It's that simple.
The 8 registers in Figure 2, as long as you know what they have to do, the specific method of use will be combined with the actual program, for everyone to explain.
Well, to sum up this section, it is very simple, 1, a Siemens S7-200PLC memory includes three kinds of memory. 2, wherein the RAM memory has 11 different registers, of which 3 are not needed for beginners.
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