Page 56 - Programmable Logic Controllers, Fifth Edition - Mobile version

utilization refers to the number of memory locations re-
quired to store each type of instruction. A rule of thumb
32 M for memory locations is one location per coil or contact.
64 K One K of memory would then allow a program contain-
1 K
ing 1000 coils and contacts to be stored in memory.
The memory of a PLC may be broken into sections
MicroLogic 1000 SLC 500 ControlLogix that have specific functions. Sections of memory used
Controller Controller Controller
1 K memory Up to 64 K memory 2 to 32 M memory to store the status of inputs and outputs are called input
Up to 20 inputs Up to 4096 inputs Up to 128,000 inputs status files or tables and output status files or tables
Up to 14 outputs and outputs and outputs (Figure 2-43). These terms simply refer to a location
where the status of an input or output device is stored.
Figure 2-41 Typical PLC memory sizes.
Each bit is either a 1 or 0, depending on whether the
input is open or closed. A closed contact would have
a binary 1 stored in its respective location in the input
1000 or in “K” increments, where 1 K is 1024 bytes of table, whereas an open contact would have a 0 stored.
memory storage (a byte is 8 bits). A lamp that is ON would have a 1 stored in its respec-
The program is stored in the memory as 1s and 0s, tive location in the output table, whereas a lamp that
which are typically assembled in the form of 16-bit is OFF would have a 0 stored. Input and output image
words. Memory sizes are commonly expressed in thou- tables are constantly being revised by the CPU. Each
sands of words that can be stored in the system; thus 2 K time a memory location is examined, the table changes
is a memory of 2000 words, and 64 K is a memory of if the contact or coil has changed state.
64,000 words. The memory size varies from as small as PLCs execute memory-checking routines to be sure
1 K for small systems to 32 MB for very large systems that the PLC memory has not been corrupted. This
(Figure 2-41). Memory capacity is an important prereq- memory checking is undertaken for safety reasons.
uisite for determining whether a particular processor will It helps ensure that the PLC will not execute if memory
handle the requirements of the specific application. is corrupted.
Memory location refers to an address in the CPU’s
memory where a binary word can be stored. A word usu- 2.8 Memory Types
ally consists of 16 bits. Each binary piece of data is a bit
and eight bits make up one byte (Figure 2-42). Memory Memory can be placed into two general categories: vola-
tile and nonvolatile. Volatile memory will lose its stored
information if all operating power is lost or removed.
Volatile memory is easily altered and is quite suitable for
Bit
most applications when supported by battery backup.
00 0 1 1 0 0 00 0 0 1 1 0 00 Nonvolatile memory has the ability to retain stored
Byte information when power is removed accidentally or in-
Word
tentionally. As the name implies, programmable logic
Figure 2-42 Memory bit, byte, and word. controllers have programmable memory that allows users

Input
module
Output
Processor memory Processor memory module
Input image table Output image table
Closed Output OFF
0000000000000000
Open 0000000000000010 0000000000000000
0000000000000000 0000000000000000
Input devices 0000000000000000 0000000000000000
0000000000000000 0000000010000000 Output ON
0000000000000000

Figure 2-43 Input and output tables.

PLC Hardware Components Chapter 2 37

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