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Programming A PLC

STEP 7-Micro/WIN32 is the program software used with the S7-200 PLC to create the PLC operating program. STEP 7 consists of a number of instructions that must be arranged in a logical order to obtain the desired PLC operation. These instructions are divided into three groups: standard instructions, special instructions, and high-speed instructions.

Standard InstructionsStandard instructions consists of instructions that are found in most programs. Standard instructions include; timer, counter, math, logical, increment/decrement/invert, move, and block instructions. Special instructions are used to manipulate data. Special instructions include shift, table, find, conversion, for/next, and real-time instructions. High-speed instructions allow for events andinterrupts to occur independent of the PLC scan time. These include high-speed counters, interrupts, output, and transmit instructions. It is not the purpose of this text to explain all of the instructions and capabilities. A few of the more common instructions necessary for a basic understanding of PLC operation will be discussed. PLC operation is limited only by the hardware capabilities and theingenuity of the person programming it. Refer to the SIMATIC S7-200 Programmable Controller System Manual for detailed information concerning these instructions. 41

Special Instructions

High-Speed Instructions

Micro/WIN32

The programming software can be run Off-line or On-line. Off-line programming allows the user to edit the ladder diagram and perform a number of maintenance tasks. ThePLC does not need to be connected to the programming device in this mode. On-line programming requires the PLC to be connected to the programming device. In this mode program changes are downloaded to the PLC. In addition, status of the input/output elements can be monitored. The CPU can be started, stopped, or reset.

Symbols

In order to understand the instructions a PLC is to carry out, anunderstanding of the language is necessary. The language of PLC ladder logic consists of a commonly used set of symbols that represent control components and instructions. One of the most confusing aspects of PLC programming for first-time users is the relationship between the device that controls a status bit and the programming function that uses a status bit. Two of the most common programmingfunctions are the normally open (NO) contact and the normally closed (NC) contact. Symbolically, power flows through these contacts when they are closed. The normally open contact (NO) is true (closed) when the input or output status bit controlling the contact is 1. The normally closed contact (NC) is true (closed) when the input or output status bit controlling the contact is 0.

Contacts

42Coils

Coils represent relays that are energized when power flows to them. When a coil is energized, it causes a corresponding output to turn on by changing the state of the status bit controlling that output to 1. That same output status bit may be used to control normally open and normally closed contacts elsewhere in the program.

Boxes

Boxes represent various instructions orfunctions that are executed when power flows to the box. Typical box functions are timers, counters, and math operations.

Entering Elements

Control elements are entered in the ladder diagram by positioning the cursor and selecting the element from a lists. In the following example the cursor has been placed in the position to the right of I0.2. A coil was selected from a pull-down list andinserted in this position.
Network 1 I0.0 I0.1 Q0.0

Network 2 I0.2 Cursor

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An AND Operation

Each rung or network on a ladder represents a logic operation. The following programming example demonstrates an AND operation. Two contact closures and one output coil are placed on network 1. They were assigned addresses I0.0, I0.1, and Q0.0. Note that in the statement list a new logic...