-2 P L C S IZES AND S COPES OF A P P L I C AT I O N S

2 1 -2 P L C S IZES AND S COPES OF A P P L I C AT I O N S

Prior to evaluating the system requirements, the designer should understand the different ranges of programmable controller products and the typical features found within each range. This understanding will enable the designer to quickly identify the type of product that comes closest to matching the application’s requirements.

Figure 21-1, previously presented in Chapter 1, illustrates PLC product ranges divided into five major areas with overlapping boundaries. The basis for this product segmentation is the number of possible inputs and outputs the system can accommodate (I/O count), the amount of memory available for the application program, and the system’s general hardware and software structure. As the I/O count increases, the complexity and cost of the system

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Guidelines 21

also increase. Similarly, as the system complexity increases, the memory capacity, variety of I/O modules, and capabilities of the instruction set increase as well.

Complexity and Cost

I/O Count Figure 21-1. PLC product ranges.

The shaded areas in Figure 21-1, labeled A, B, and C, reflect the possibility of controllers with enhanced (not standard) features for a particular range. These enhancements place the product in a gray area that overlaps the next higher range. For example, because of its I/O count, a small PLC would fall into area 2, but it could have analog control functions that are standard in medium-sized controllers. Thus, this type of product would belong in area A. Products that fall into these overlapping areas allow the user to select the product that best matches the application’s requirements, without having to select the larger product, unless it is necessary. The following discussion presents information about the five PLC categories, as well as the overlap- ping categories.

S EGMENT

1: M ICRO PLC S

Micro PLCs are used in applications that require the control of a few discrete I/O devices, such as small conveyor controls. Some micro PLCs can perform limited analog I/O monitoring functions (e.g., monitoring a temperature set point or activating an output). Figure 21-2 shows a typical microcontroller, while Table 21-1 lists the standard features of micro PLCs.

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Figure 21-2. PLC Direct’s micro PLC DL105.

• U p t o 3 2 I / O • 1 6 - b i t p r o c e s s o r • R e l a y r e p l a c e r • M e m o r y u p t o 1 K • D i g i t a l I / O • B u li t - i n I / O s i n a c o m p a c t u n i t • M a s t e r c o n t r o l r e l a y s • T i m e r s a n d c o u n t e r s

• P r o g r a m m e d w i t h h a n d h e l d p r o g r a m m e r Table 21-1. Standard features of micro PLCs.

S EGMENT

2: S MALL PLC S

Small controllers are mostly used in applications that require ON/OFF control for logic sequencing and timing functions. These PLCs, along with microcontrollers, are widely used for the individual control of small ma- chines. Often, these products are single-board controllers. Table 21-2 lists the standard features of small PLCs.

Area A. Area A includes controllers that are capable of having up to 64 or 128 I/O, along with products that have features normally found in medium-sized controllers. The enhanced capabilities of these small controllers allow them to be used effectively in applications that need only a small number of I/O, yet require analog control, basic math, I/O bus network interfaces, LANs, remote I/O, and/or limited data-handling capabilities (see Figure 21-3). A typical application of an area A controller is a transfer line in which several small machines, under individual control, must be interlocked through a LAN.

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Guidelines 21

• P r o g r a m m e d w i t h h a n d h e l d p r o g r a m m e r Table 21-2. Standard features of small PLCs.

Figure 21-3. Area A (SLC500) controller capable of handling up to 72 discrete and 4

analog I/O.

S EGMENT

3: M EDIUM PLC S

Medium PLCs (see Figure 21-4) are used in applications that require more than 128 I/O, as well as analog control, data manipulation, and arithmetic capabilities. In general, the controllers in segment 3 have more flexible hardware and software features than the controllers previously mentioned. Table 21-3 lists these features.

Area B. Area B contains medium PLCs that have more memory, table- handling, PID, and subroutine capabilities than typical medium-sized PLCs,

as well as more arithmetic and data-handling instructions. Figure 21-5 shows

a PLC that falls into this category.

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Figure 21-4. Medium-sized PLC 5/11

Figure 21-5. Omron’s area B CV500 PLC

(left) and PLC 5/20 (right) processors

with a temperature control module (up to

with up to 512 I/O capacity.

1024 I/O).

• S u p p o r t I / O b u s n e t w o r k s Table 21-3. Standard features of medium PLCs.

S EGMENT

4: L ARGE PLC S

Large controllers (see Figure 21-6) are used for more complicated control tasks, which require extensive data manipulation, data acquisition, and reporting. Further software enhancements allow these products to perform complex numerical computations. Table 21-4 summarizes the standard features of large PLCs.

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Figure 21-6. Large Mitsubishi A3NCPU controller with 2048 I/O capacity.

• S u p p o r t I / O b u s n e t w o r k s Table 21-4. Standard features of large PLCs.

Area C. Area C includes the segment 4 PLCs that have a large amount of application memory and I/O capacity. The PLCs in this area also have greater math and data-handling capabilities than other large PLCs. Figure 21-7 shows an example of this type of controller.

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Figure 21-7. Giddings & Lewis’s area C PIC900 with up to 3168 I/O and motion I/O, IEC programming, and floating-point math capabilities.

S EGMENT

5: V E RY L ARGE PLC S

Very large PLCs (see Figure 21-8) are utilized in sophisticated control and data acquisition applications that require large memory and I/O capacities. Remote and special I/O interfaces are also standard requirements for this type of controller. Typical applications for very large PLCs include steel mills and refineries. These PLCs usually serve as supervisory controllers in large, distributed control applications. Table 21-5 lists standard features found in segment 5 PLCs.

Figure 21-8. Very large PLC-3 from Allen-Bradley with 8190 I/O capability.

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Table 21-5. Standard features of very large PLCs.