Beginner PLC ladder programming should be to follow certain rules, and develop good habits. In this paper, Mitsubishi FX series PLC, for example, briefly explain the Mitsubishi PLC ladder programming rules to be followed, allowing users to better understand how to learn to see the Mitsubishi PLC ladder programming, we want to help. One thing to note is that although this article Mitsubishi PLC, for example, but these rules when other PLC program must also be observed.
1, PLC ladder ladder are starting left bus, and finally the right bus (usually can be omitted without painting, painting only left bus). The left side of each line is a combination of contacts, it represents the logical drive coil conditions, and represents the result of logic coil can only be connected on the right bus. Contact can not appear in the right side of the coil. The following figure (a) should read (b):
2, the contact should be plotted on the horizontal line, vertical line should be drawn on the relationship as shown in (a) contacts and other contacts between the X005 does not recognize. For such a bridge circuit should be from left to right, top to bottom, the principle of one-way, single draw all the way. FIG. (B) below:
3, parallel block series should contact many of the path on the ladder to the left (left and right light-weight principle); parallel blocks in series, parallel path should be more contacts on the top of the ladder (top heavy the rules). To do so, the program is simple, thus reducing scan time directive, which is particularly important for some large programs. As shown below:
4, not to use double-coil output. If the same ladder, more than two coils of the same component or twice, then known as the reuse or dual-coil output coil. Usually one of the dual-coil output ladder for beginners easy to commit wrong. In the double-coil output, only the last coil to be effective, and front coil is invalid. This is caused by the PLC scan feature of the decision.
PLC uses the CPU work cycle scan. Generally consists of five phases (as shown): internal diagnostics and treatment, and communication peripherals, input sample, the user program execution and output refresh. When the mode switch is in STOP, before performing only two phases: internal diagnostics and treatment, and communications peripherals.
1, the input sampling stage
PLC sequential reads the state of each input, and the input image is stored in the register we call the internal unit. When entering the program execution stage, such as the input changes state corresponding cell information input image area does not change with it, only the next input sampling phase of the scan cycle, the input image area corresponding to the cell information will change. Therefore, PLC will be overlooked pulse variation is less than the amount of the scan cycle switch input.
2, program execution stage
PLC from the program step 0, beginning with the first following the first left and right sequence scans the user program and logical operations. PLC input image by content area for logical operations, and the result is written to the output image area, rather than directly to the output terminals.
3, the output refresh period
PLC output terminal changes depending on the content of the output image area. This is the actual output of the PLC.
Above brief description of the working principle of PLC, the following examples explain why we then to write ladder program should not be repeated using a coil. As shown below, when the reset input is sampled input image area X001 = ON, X002 = OFF, Y003-ON, Y004 = ON is actually written to the output image area. But to continue down, because X002 = OFF, so Y003 = OFF, this is after the result has been written to the output image area, change the original state of Y003. Therefore, the output refresh period, the actual external output Y003 = OFF, Y004 = ON. Many newcomers to come across such a problem, why X001 has been closed, and no output Y003 it? Logic is wrong. In fact, because the use of double coil caused.
Note: we are talking about is not (should not) use dual-coil, double coils are not absolutely prohibited, in some special occasions can also use double-coil, then you will need a more extensive programming the experience and skills. Let's talk about this. But still do not risk it for beginners. In fact, it can be seen from the above examples, the reason will result in Reuse coil output Y003 confusion is due to the reason the program is executed sequentially from top to bottom caused. But if we can change the order of program execution, at any time to ensure that the two coils is only one logical drive occurs, you can use double-coil. The most common method is to use a jump instruction
Program Analysis: M0 closed, the program jumps to at P0 (no X001 statement), M0 normally closed off, CJ P1 does not occur, the next statement. In this case, Y003 to X002 state drive. State M0 disconnected, according to the order of execution of T003 X001 is driven M0 NC closed skip P1 state by X003 to drive on Y004, which skips the X002 drive Y003 statements. Visible at the same time, Y003 drive only one can occur. In this case, the use of double-coil is possible.
But in ladder programming, we still have to try to avoid the use of dual-coil, and the introduction of auxiliary relay is a commonly used method. As shown below:
Figure (b),, X001 and X002 contact control auxiliary relay M000, X003 ~ X005 contact control auxiliary relay M001, and then by two relays M000, M001 parallel combination of contacts to control the coil Y000. Such logic has not changed, yet the double coil into a single coil.
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