3 -4 PLC C I RCU I T S AN D L OGI C C ON TACT S Y M BOLOGY

3 -4 PLC C I RCU I T S AN D L OGI C C ON TACT S Y M BOLOGY

Hardwired logic refers to logic control functions (timing, sequencing, and control) that are determined by the way devices are interconnected. In contrast to PLCs, in which logic functions are programmable and easily changed, hardwired logic is fixed and can be changed only by altering the way devices are physically connected or interwired. A prime function of a PLC is to replace existing hardwired control logic and to implement control func- tions for new systems. Figure 3-10a shows a typical hardwired relay logic circuit, and Figure 3-10b shows its PLC ladder diagram implementation. The important point about Figure 3-10 is not to understand the process of changing from one circuit to another, but to see the similarities in the representations. The ladder circuit connections of the hardwired relay circuit are implemented in the PLC via software instructions, thus all of the wiring can be thought of as being inside the CPU (softwired as opposed to hardwired).

PB4 START

All OL's3

M3

PB10

PB11 START

S2 SWITCH

All OL's8

Figure 3-10a. Hardwired relay logic circuit.

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

S ECTION Introductory Logic C HAPTER 1 Concepts

Concepts 3

The logic implemented in PLCs is based on the three basic logic functions (AND, OR, and NOT) that we discussed in the previous sections. These functions are used either alone or in combination to form instructions that will determine if a device is to be switched on or off. How these instructions are implemented to convey commands to the PLC is called the language. The most widely used languages for implementing on/off control and sequencing are ladder diagrams and Boolean mnemonics, among others. Chapter 9 discusses these languages at length.

The most conventional of the control languages is ladder diagram. Ladder diagrams are also called contact symbology, since their instructions are relay-equivalent contact symbols (i.e., normally open and normally closed contacts and coils).

0 1 2 30 M1 OL's1 0 30

All 5 10 11 34 M3 OL's3

All 12 13 35 M1 OL's4

37 40 M6 OL's6 15 40

37 41 M7 OL's7 41

37 42 M8 OL's8 42

Figure 3-10b. PLC ladder diagram implementation of Figure 3-10a.

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

S ECTION Introductory Logic C HAPTER 1 Concepts

Concepts 3

Contact symbology is a very simple way of expressing control logic in terms of symbols that are used on relay control schematics. If the controller language is ladder diagram, the translation from existing relay logic to programmed logic is a one-step translation to contact symbology. If the language is Boolean mnemonics, conversion to contact symbology is not required, yet is still useful and quite often done to provide an easily under- stood piece of documentation. Table 3-6a, shown later, provides examples of simple translations from hardwired logic to programmed logic. Chapter 11 thoroughly explains these translations.

The complete ladder circuit, in Figure 3-10, shown earlier, can be thought of as being formed by individual circuits, each circuit having one output. Each of these circuits is known as a rung (or network); therefore, a rung is the contact symbology required to control an output in the PLC. Some controllers allow a rung to have multiple outputs, but one output per rung is the convention. Figure 3-11a illustrates the top rung of the hardwired circuit from Figure 3-10, while Figure 3-11b shows the top rung of the equivalent PLC circuit. Note that the PLC diagram includes all of the field input and output devices connected to the interfaces that are used in the rung. A complete PLC ladder diagram program, then, consists of several rungs. Each rung controls an output interface that is connected to an output device, a piece of equipment that receives information from the PLC. Each rung is a combination of input conditions (symbols) connected from left to right between two vertical lines, with the symbol that represents the output at the far right.

PB4 START

0 1 2 30 M1 OL's1 0 30

Figure 3-11. (a) Top rung of the hardwired circuit from Figure 3-10 and (b) its

equivalent PLC circuit.

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

S ECTION Introductory Logic C HAPTER 1 Concepts

Concepts 3

The symbols that represent the inputs are connected in series, parallel, or some combination to obtain the desired logic. These input symbols represent the input devices that are connected to the PLC’s input interfaces. The input devices supply the PLC with field data. When completed, a ladder diagram control program consists of several rungs, with each rung controlling a specific output.

The programmed rung concept is a direct carryover from the hardwired relay ladder rung, in which input devices are connected in series and parallel to control various outputs. When activated, these input devices either allow current to flow through the circuit or cause a break in current flow, thereby switching the output devices ON or OFF. The input symbols on a ladder rung can represent signals generated by connected input devices, connected output devices, or outputs internal to the controller (see Table 3-4).

P li o t il g h t

Table 3-4. ON/OFF input and output devices.