Understanding Shift Registers in Mitsubishi PLC: SFR, SFL, BSFR, and BSFL Instructions

Understanding Shift Registers in Mitsubishi PLC: SFR, SFL, BSFR, and BSFL Instructions

In digital electronics, a shift register is a type of register that allows the contents of its bits to be moved (shifted) left or right. This functionality is useful in various control systems, where data needs to be moved or manipulated in a sequence. When we bring this concept into the realm of Mitsubishi PLC programming, there are specialized instructions such as SFR, SFL, BSFR, and BSFL that can be used to shift bits according to the specific requirements of the control process.

Let's explore these instructions and see how they are applied in Mitsubishi PLCs.

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1. SFR and SFL Instructions in Mitsubishi PLC

In Mitsubishi PLCs, the SFR (Shift Right) and SFL (Shift Left) instructions are used to shift bits within a 16-bit word. These instructions allow you to manipulate data in a register (such as D10) by shifting bits either to the right or left.

• SFR (Shift Right): This instruction shifts the bits of a 16-bit word to the right by a specified number of bits. Each shift to the right results in the least significant bit (rightmost bit) being shifted out, and the leftmost bit being filled with zero.
• SFL (Shift Left): Similarly, this instruction shifts the bits of a 16-bit word to the left by a specified number of bits. Each shift to the left moves the most significant bit (leftmost bit) out, and the rightmost bit is filled with zero.

The number of bits to shift is specified by a constant (denoted as K1, K2, etc.) in the ladder diagram.

Example of SFR and SFL in Ladder Logic

Consider a simple scenario where we use SFR and SFL instructions with a 16-bit register like D10. Let’s break it down:

SFR Instruction (Shift Right):
        The SFR instruction will shift the contents of the D10 register to the right by the number of bits             specified. For example, if we specify K1 as the number of bits to shift, the contents of D10 will             move to the right by one position. Every time this instruction is executed, the data in D10 will be         shifted to the right by the specified number of bits.
        LD M0               // If M0 is ON, the shift will happen
        SFR D10 K1          // Shift D10 to the right by K1 (1 bit).

 SFL Instruction (Shift Left):
        The SFL instruction works similarly to the SFR instruction, but instead of shifting the bits to the           right, it shifts the bits to the left.. If K1 is used to specify the number of bits to shift, each                       execution          will move the contents of D10 to the left by one position.    
         LD M1               // If M1 is ON, the shift will happen
         SFL D10 K1          // Shift D10 to the left by K1 (1 bit)\

Practical Example: Shifting Bits in Action

Let’s look at a practical example to better understand how the SFR and SFL instructions work.

Imagine that we have a 16-bit register (D10) with the following initial value:

Scenario 1: Using M0 to Shift Right

When you press M0 once, the SFR instruction will shift the bits of D10 to the right by K1 (1 bit). Here’s what happens:

• Initial State of D10:
• 1000001000000000
• After pressing M0 once (shift right by 1 bit):
• 0100000100000000

Scenario 2: Pressing M0 Multiple Times

If you press M0 four more times, the bits will continue shifting to the right each time. The sequence would look like this:

• After 1st press (shift right by 1 bit):
• 0010000010000000
• After 2nd press (shift right by another bit):
• 0001000001000000
• After 3rd press (shift right by another bit):
• 0000100000100000
• After 4th press (shift right by another bit):
• 0000010000010000

As you can see, with each press of M0, the bits move one position to the right.

Scenario 3: Using M1 to Shift Left

Now, let’s say we want to shift the bits of D10 back to the left. If you press M1, the SFL instruction will shift the bits of D10 to the left by K1 (1 bit) for each press.

• Initial State of D10 (after shifting right):
• 0000010000010000
• After pressing M1 once (shift left by 1 bit):
• 0000100000100000

BSFR and BSFL Instructions: Bit-Specific Shifting

Mitsubishi PLCs also provide BSFR and BSFL instructions, which are similar to SFR and SFL, but operate on specific bits rather than on entire 16-bit words. These instructions are used to shift individual bits of a bit array, allowing for more granular control of data.

• BSFR (Bit Shift Right): Shifts individual bits in a bit array to the right.
• BSFL (Bit Shift Left): Shifts individual bits in a bit array to the left.

These instructions are useful when you want to manipulate individual bits in a more detailed manner, such as in a bit pattern or for controlling specific outputs based on bit-level operations.

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In Mitsubishi PLC programming, SFR, SFL, BSFR, and BSFL are essential instructions for performing bit shifting operations. Whether you're dealing with a 16-bit word or working with individual bits, these instructions allow you to manipulate and control data efficiently. Understanding how to use these shift register instructions in Mitsubishi PLCs can greatly enhance your ability to develop complex and effective control systems.

• SFR and SFL are used for shifting entire 16-bit words to the right or left.
• BSFR and BSFL are used for bit-level shifting, offering more detailed control.

By incorporating these instructions into your ladder logic, you can achieve precise timing and control for a variety of industrial automation applications.