Siemens PLC programming software... MOV_W, MOV_B, SHR_B are all meanings - Database & Sql Blog Articles

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What do the terms MOV_W, MOV_B, and SHR_B mean in Siemens PLC programming software, and what are the functions of their pins?

In Siemens PLC programming, instructions like MOV_B (Move Byte), MOV_W (Move Word), and SHR_B (Shift Right Byte) are commonly used for data manipulation. The Move Byte instruction (MOV_B) transfers the value from an input byte (IN) to an output byte (OUT) without modifying the original data. Similarly, the Move Word instruction (MOV_W) copies a 16-bit word from the input to the output location.

The Shift Right Byte instruction (SHR_B) shifts the bits of the input byte (IN) to the right by a specified number of positions (N). Each shifted bit is replaced with a zero. If the shift count (N) is equal to or greater than 8, the maximum number of shifts allowed is 8. This ensures that the entire byte is cleared when shifting more than 8 times. A similar instruction, SLB (Shift Left Byte), performs the same operation but shifts bits to the left instead.

When working with ladder logic diagrams, there are several important rules to follow. First, output coils (Y) cannot be directly connected to the left bus. Input contacts (X) should not be directly connected to the right bus either. Normally open contacts should always come before normally closed contacts in a rung. Additionally, self-locking contacts of output coils must be placed in parallel with normally open contacts to maintain the state of the output.

Each output coil can only be used once in a program, but it can have multiple contacts associated with it. Input contacts, on the other hand, can be used as many times as needed. Internal relays (R) and timers (T) can also be used once per coil, but they can have unlimited contact points. This allows for efficient use of internal memory and logical control within the PLC program.

PLCs operate using a scan cycle, where they process one input signal at a time during each scan. This means that the PLC reads all input signals first, processes the logic, and then updates the outputs. This scanning method ensures consistent and predictable behavior in industrial automation systems.

M is an internal relay, which can be categorized into general-purpose and special-purpose types. These relays are used to store intermediate values or control internal logic within the program. T represents a timer, which is used to manage time-based operations. Timers can be of different types, such as standard timers or power-off retention timers, depending on the specific application requirements.