Detailed explanation of S7-200SMART series PLC bit logic operation instructions and bit read and write operations

Siemens S7-200 SIMATIC PLC instruction list

Detailed explanation of S7-200SMART series PLC bit logic operation instructions and bit read and write operations

The bit logic instructions of Siemens s7-200SAMRT are the most frequently used in PLC. Except for some very special program segments, it can be said that every ladder diagram program segment cannot do without bit logic instructions. As the name implies, the object of the bit logic instruction operation is the binary bit in the register storage unit. The essence of the bit logic operation instruction is to find the specified bit through bit addressing, read or change the state of the bit (0 or 1). The logical operations between bits are mainly AND, OR, and NOT operations.

1. Reading and writing bits. In the ladder diagram, reading bits means placing contacts, and writing bits means placing coils. As shown in the figure below, in the first program segment, we first read the binary bit I0.0 (that is, the 0th bit of every 0th byte of our process image input register) by placing contact I0.0. The corresponding object is input I0.0, and its state is written to M0.1 of the flag register. The second program segment is to copy the current value of M0.1 to Q0.0. So we have to remember which contact to place to read the bit, and which coil to place to write a bit.

Programming Examples

In the STL instruction, place the contact, use the LD instruction, LOAD means, LD I0.0 means to read the value of I0.0 and load it into the stack. = M0.1 means to make the value of M0.1 equal to the value at the top of the current stack.

STL directives

2. AND operation, series connection is AND operation. A and B, when AB are both 1, the operation result is 1, otherwise it is 0. As shown in the figure below: SM0.0 and M0.1 are connected in series for AND operation, and the operation result is written into Q0.0. If SM0.0 and M0.1 are both 1, Q0.0=1, if either SM0.0 or M0.1 is 0, Q0.0=0.

3. OR operation, parallel connection is OR operation. A or B, the operation result is 0 only when A and B are both 0, otherwise it is 1. As shown in the figure below: I0.0 and I0.1 are connected in parallel to do AND operation, and the operation result is written into Q0.1. If the values ​​of I0.0 and I0.1 are both 0, then Q0.1=0, if any bit state of I0.0 and I0.1 is 1, then Q0.1=1.

AND operation

In STL instructions, A XX.X is an AND operation, and O XX.X is an OR operation.

4. Negation operation, which is negation, turns 1 into 0 and 0 into 1. The program in the figure below is to perform the OR operation first and then negate it.

5. Rising and falling edge instructions, positive jump contact instructions (rising edge) allow energy to flow for one scan cycle after each disconnection to connection transition. Negative jump contact instructions (falling edge) allow energy to flow for one scan cycle after each connection to disconnection transition.

6. Set and reset instructions, Set (S) and Reset (R) instructions are used to set (turn on) or reset (turn off) a group of bits (N) starting from a specified address (bit). You can set or reset 1 to 255 bits. If the reset instruction specifies a timer bit (T address) or a counter bit (C address), the instruction will reset the timer or counter bit and clear the current value of the timer or counter.