This document describes different types of memory used in programmable logic controllers (PLCs). It discusses process-image input and output registers used to transfer data between the PLC and external devices. It also describes variable memory for storing data, sequence control relays for organizing machine operations, and special memory bits for communicating information. Local memory is provided for scratchpad use or passing parameters. Analog inputs convert real-world values to digital values, while analog outputs convert digital values back to proportional currents or voltages. The document concludes by explaining how to convert a relay-based logic diagram into a ladder logic program for a PLC using basic logic elements and their addressing.

1. Memory types
 Process-image input register (I)
Format:
Bit
Byte, Word, Double Word

I[byte address].[bit address] I0.1
I[size][starting byte address] IB4
Process-image output register (Q)
Format:
Bit
Byte, Word, Double Word

Q[byte address].[bit address] Q1.1
Q[size][starting byte address] QB5
Variable memory area (V)
You can use V memory to:
store intermediate results of the control logic operations. •
store other data pertaining to your process or task. •
Format:
Bit
Byte, Word, Double Word
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V[byte address].[bit address] V10.2
V[size][starting byte address] VW100

2. Memory types
 Sequence control relay area (S)
They are used to organize machine operations or steps into equivalent
program segments. SCRs allow logical segmentation of the control
Format:
Bit
S[byte address].[bit address] S3.1
Byte, Word, Double Word S[size][starting byte address] SB4

Special memory bits (SM)
The SM bits provide a means for communicating information between the
CPU and your program. You can use these bits to select and control some
of the special functions of the S7-200 CPU, such as:
• A bit that turns on for the first scan cycle
• Bits that toggle at fixed rates
• Bits that show the status of math or operational instructions
Format:
Bit
SM[byte address].[bit address] SM0.1
Byte, Word, Double Word SM[size][starting byte address] SMB86
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3. Memory types
 Local memory area (L)
The S7-200 PLCs provide 64 bytes of local (L) memory of which 60 can be
used as scratchpad memory or for passing formal parameters to subroutines.
Format:
Bit
L [byte address].[bit address] L0.0
Byte, Word, Double Word
L [size] [starting byte address] LB33
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4. Memory types
 Analog inputs (AI)
The S7-200 converts a real-world, analog value (such as temperature
or voltage) into a word-length (16-bit) digital value. You access these
values by the area identifier (AI), size of the data (W), and the starting
byte address. Since analog inputs are words and always start on
even-number bytes (such as 0, 2, or 4), you access them with evennumber byte addresses (such as AIW0, AIW2, or AIW4),as shown in
Figure Analog input values are read-only values.
Format: AIW [starting byte address] AIW4
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5. Memory types
•Analog outputs (AQ)
The S7-200 converts a word-length (16-bit) digital value into a current
or voltage, proportional to the digital value (such as for a current or
voltage). You write these values by the area identifier (AQ), size of the
data (W), and the starting by address. Since analog outputs are words
and always start on even-number bytes (such as 0, 2, or 4), you write
them with even-number byte addresses (AQW0, AQW2, AQW4),
Format: AQW [starting byte address] AQW4
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6. Replacing Relay by PLC
First step- We have to translate all of the items
we're using into symbols the plc understands
A contact symbol
A coil symbol
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7. Second step
- We must tell the plc where everything is
located. In other words we have to give all
the devices an address.
Final step
- We have to convert the schematic into a
logical sequence of events.
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8. Ladder Diagram and Programming:
Load: The load (LD) instruction is a normally open contact
A Load (contact) symbol
LoadBar: The LoadBar instruction is a normally closed contact.
A LoadBar (normally closed contact) symbol
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9. Out :The Out instruction is sometimes also called an Output Energize instruction.
The output instruction is like a relay coil
An OUT (coil) symbol
OutBar: The outbar instruction is like a normally closed relay coil
An OUTBar (normally closed coil) symbol
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10. Logic elements
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