PLc Memory & Addressing

1. PLC
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PLC MEMORY
 PLC Memory consists of the operating system and firmware of
the processor and modules, and the program and data that is
used by the programmer.
 Though the program can be saved on Flash or SD RAM cards
without a battery, the data exchange rate is too slow to use this
for the actual interfacing of the program with its data.
 There are volatile and non-volatile areas of memory, and that
the volatile part of memory needs a battery, “super-capacitor”
or other rechargeable energy storage module to hold its
program and/or data.
 The operating system itself on a processor is held in non-volatile
memory, called “firmware”.
 To change the firmware on a PLC a “Flash” program or tool
needs to be used to download it. This is usually included with
the programming software.
 I/O, communications and other modules also often have
firmware built in.
 The firmware update tools can also update these modules and
the firmware is usually available from the manufacturer’s
website. It is necessary to have software that is at least as up-to-
date as the firmware being installed.
 Ladder logic programs are stored in RAM.
 When a new ladder logic program is loaded into a PLC’s
memory, the old program that was stored in the same locations
is over-written and essentially erased.
 The memory capacities of PLCs vary. Memory capacities are
often expressed in terms of kilobytes (K).
 One byte is a group of 8 bits.
 One bit is a memory location that may store one binary number
that has the value of either 1 or 0. (Binary numbers are
addressed in Module 2).
 1K memory means that there are 1024 bytes of RAM.
 16K memory means there are 16 x 1024 =16384 bytes of RAM

2. PLC
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RAM” (Random Access Memory)
 Program and data memory in a PLC is contained in “RAM”
(Random Access Memory).
 The RAM part of memory in a PLC can be separated into two
general areas; Program Memory and Data Memory.
 Program memory consists all of the lists of Instructions and
Program Code. This is what is sent to the processor.
 Data memory includes the Input and Output Image Tables as
well as Numerical and Boolean Data.
 Data memory may be lost when a processor loses power.
Volatile – memory = Data will be lost when the power cut off.
Non Volatile – memory = Data will NOT be lost when the power cut
off.
ROM (READ ONLY MEMORY)
 ROM is used to store programs and data that should not be
altered.
 For example, the PLC’s operating programs are stored in
ROM.
8 bit = 8b 1 Byte = 1B
2 Byte = 16 bit 1 word
1024 bits = 1024b 1 Kilo Byte=1KB
1024 KB 1 MB
1024 MB 1 GB
1024 GB 1 TB

3. PLC
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PLC ADDRESSING
Type Address Example
Input i5.01
Output Q3.04
Memory M2.07
SEE THE EXAMPLE PLC PROGRAM.
Once you press the start button motor should start.
When you press the stop button motor should stop.
1. Create address allocation Table?
2. Create LADDER logic program
Answers
Address allocation Table
Type Address
Start button input I0.00
Stop button Input I0.01
Motor Output Q10.00
LADDER Logic program

4. PLC
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Try this in PLC simulator APP.
In the next session, we will see latching LAD program.
Watch these video
https://www.youtube.com/watch?v=zsajTNtxfAE
https://www.youtube.com/watch?v=Ei4_HqzUFBs
https://www.youtube.com/watch?v=gcXR6BWb_ag
https://www.youtube.com/watch?v=Hci-eW5IihM