Basic Operating Principles of PLCs
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The battery allows the retention of the automation program in the RAM memory, even when
the power supply has been interrupted, and it is possible to equip the PLC with an extra memory
of EEPROM type (see Section 6.8.2) for permanent storage of the automation program.
6.8.2 Memory Types
As has been shown up to now, there are various types of memories that a PLC utilizes (e.g., ROM,
RAM, etc.) In this section, the most common memory types found in a PLC, will be described,
including their typical characteristics and their specific utilization in the PLCs.
6.8.2.1 Read Only Memory (ROM)
The non-volatile memories (ROM) are used for the permanent storage of functional data and
programs, so that only the action of reading of the memory contents is allowed. In general, the
information stored in a ROM memory of a PLC is put there by the manufacturer, and is mainly
related to the way that the PLC operates. Specifically, the operating system of the PLC is stored in
the ROM memory and contains the set of instructions for operating the PLC. The ROM instruc-
tions cannot be erased, which means that they have the ability to retain their stored data, even in
the case of a power supply cut, thus they do not require an emergency power supply.
6.8.2.2 Random Access Memory (RAM)
RAM memories are sometimes referred to as read or write memories, and are designed in a way
that data and programs can be directly written or read from, without restrictions. These memories
consist of the most flexible type of memory, since the user is able to access all of the memory loca-
tions and add new data directly by erasing the previously written information as many times as
needed. In the RAM memory of a PLC, the data are stored electrically through the programming
device and, in some cases, it is not needed to stop the operation of the PLC. Usually in the RAM
memory, the automation program of the user is stored as well as any other data that are produced
or acquired during the execution of this program.
In a potential power supply loss, even an instant one, the RAM memory loses all of its con-
tents. This means that in a case of power failure, the PLC will lose the automation program and
thus it is necessary to load the program again. To deal with this case, RAM memory is combined
with a battery that can provide necessary power to the memory for retaining the current data for
a time window of about one year.
6.8.2.3 Erasable Programmable Read Only Memory (EPROM)
Today, EPROM memory is not used in computers and PLCs and has been replaced by EEPROM
chips. We refer to this kind of memory only because an engineer may encounter it in old PLCs still
operating in some industries. The EPROM memory, in contrast to its name, is a ROM type memory
that can be reprogrammed after a complete erase by the application of ultraviolet radiation. Unlike
RAM, EPROM does not lose its stored information in a potential power loss, and it does not allow user
access to change the information already stored there. The integrated circuit of the EPROM memory
carries a small window over the memory position, and when ultraviolet radiation is transmitted there
for a few minutes, the contents of the memory are erased. This process demands to remove the EPROM
memory from the PLC and to transfer it to a specific erasing unit. This process is an overall disadvan-
tage, since the operation of the PLC needs to be terminated. After erasing the EPROM, a full automa-
tion program can be stored to the memory through the utilization of the programming device.
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Introduction to Industrial Automation
6.8.2.4 Electrically Erasable Programmable Read Only Memory (EEPROM)
The EEPROM memory is a type of memory that can be erased by the application of an electri-
cal voltage through proper electrical connections to the memory’s IC through the programming
device. After this erase, the EEPROM memory can be rewritten and it does not lose the stored
information in a potential power loss. Although it shows the same flexibility as RAM memory, it
is generally slower (slower data access to the stored information) than RAM memory.
When an automation program is loaded for the first time in a PLC, the testing phase of the
overall control system follows. During this time, it is quite common to require changes and altera-
tions in the automation program, changes that can be directly applied to RAM memory by the
sequential download of the modified program. After the finalization of the automation program
and its proper operation, this is transferred to the EEPROM (if such a memory is available in the
PLC) which constitutes the final and definitive memory for the normal operation of the PLC.
Many larger PLCs have the capability to accept, except from basic RAM, external memo-
ries in specific slots that are available for this purpose. These memories are usually of EEPROM
or FLASH type, can vary in capacity, and are utilized for program or data storing. A FLASH
memory is similar to an EEPROM memory, with the main differences between them concerning
the user, the access, and erase manner of the stored data. In the first one, blocks of thousands of
bytes can be erased at a time, while in the second one, a byte at a time is possible to be accessed
and erased. Finally, many PLCs can be connected to external storage devices, such as hard drives
for the storing of big automation programs or sets of data.
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