Memory

MaintenanceCircleTeam

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maintenance

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May 28th 2007

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NEWSLETTER FOR MAINTENANCE COMMUNITY Word for the day: MEMORY

With increased usage of Computers, PLCs and advanced control system in machines, design and software engineers write instructions to control the machine. These instructions are written in computer language (Visual Basic, C++, Java and many more), ladder language (Siemens’ Step 5, Omron’s Programmer, Fanuc and many more) or any specific “code” suitable for a particular machine or manufacturer. We are already entering an era of computerization where every step of manufacturing process, from receiving an order to shipping the material, is being automated and handled by computer. It is no wonder that machines and computes can perform same high speed tasks repeatedly over years. One of the important components of this entire system is MEMORY which holds vital information and machine programs. Memory can be referred to as a physical electronic device – generally called storage media – where all the vital information including programs, ladder logics, machine parameters, production data, and company information are stored (actually the list can be endless since anything you think can be stored in memory). Before getting into the concepts of memory, it is important to know that these electronic devices can only understand “0” and “1” which are known as bits. In electrical terminology, “0” refers to OFF or 0 volt condition and “1” refers to ON or 5 volt condition. Hence, any type of information be it a machine program, ladder logic, production data are stored in millions of combinations of 0s and 1s. Since any type of data, program or instruction contains alphabets, numbers and special characters, a single 0 or 1 cannot identify all of them. So a standard was evolved and eventually 8 bits together were found sufficient to identify all the characters. This group of 8 bits is called one Byte. Every letter, space, punctuation occupies one byte and hence we can measure the size of a program, instruction, data by number of bytes needed. But in reality, one byte is very small to identify any data. Table 1 gives various higher versions and a comparison to help understand better. Apart from this, pictures, images, drawings and all other information are converted into bytes before being stored in memory.

Size 1 Byte (1B)

Number of Bytes 1 Byte

1 Word (2B)

1 Kilobyte (1 KB) 1 Megabyte (1 MB) 1 Gigabyte (1 GB) 1 Terabyte (1 TB)

1 Nibble

Comparison Any alphabets, individual numbers, special characters. For instance, alphabet A and number 1 occupies 1 byte each 2 Bytes Twice the size of 1 byte. Used in PLCs to identify inputs, outputs and analog signals. 1024 Bytes Half page of purely typewritten page occupies 1 kilobyte 1024 Kilobytes or 1048576 Bytes 500 Pages 1024 Megabytes or 1048576 500,000 Pages or 1000 books of 500 Kilobytes or 1073741824 Bytes pages 1024 Gigabytes or 1048576 1000000 books of 500 pages each Megabytes or 1073741824 Kilobytes or 1099511627776 Bytes ½ Byte (4 bits) Rarely used in few PLCs, Microcontrollers for Input / Output addressing

Table 1 If you like to improvise this article or contribute or comment please mail us at: [email protected] This document contains information for reference only. We assume no responsibility for its implication.

MaintenanceCircleTeam

Page 2

May 28th 2007

With this little background, let us look at some of the commonly used electronic devices used as memory. Please bear in mind that with technology changing every moment, what is in demand today can be extinct tomorrow, but some of the storage devices have survived due to their simplicity, reliability and long life.

EEPROM – It is the short form for Electrically Erasable Programmable Read Only Memory and is one of the most commonly used memory devices used in machines to store programs. It is also used in computers to store control program of various input and output devices. This is a permanent type of memory device which means once program is written, it will NOT be erased unless a small electrical signal is given for certain duration. EEPROM is usually available in IC packaging with 32, 40, 64 pin configuration which can be removed and inserted into a slot provided on PCB. The memory capacity of single EEPROM varies from 8 Kilobytes to 64 Kilobytes. Normally, small PLC programs, machine codes are stored on EEPROMs which can be easily replaced during upgrade, troubleshooting and machine re-installation. In some machines, more than one EEPROM will be used to store large programs. EPROM (UV-PROM) – It is the short form for Erasable Programmable Read Only Memory and has features very similar to EEPROM. The only difference is that in order to erase any previously stored program, it should be exposed to Ultraviolet rays for certain duration. Even by exposing to sunlight for long time, the program gets erased and hence care should be taken while removing these devices from PCB. FLOPPY DISKS – Available in 3 ½” size – during 80s and 90s, 5 ¼” size was also available but has become obsolete now – it can store 1.44 Megabytes of information. This is one of the magnetic types of storage devices which are similar to audio or video cassette tapes. The reliability is very good and can store information for long time. Machine programs, production data, and other commonly used information are stored on floppy disks which can be downloaded as and when needed. HARD DISK DRIVES (HDD) – This is the most widely used and reliable type of storage devices which forms an integral part of every computer and many sophisticated machines. Hard disk drives are quite similar to floppy disks except that they are built mechanically stronger to last longer and hold precious data for long time. HDDs are capable of storing data up to 2 terabytes. 60GB, 80GB, 120GB are commonly used sizes available today. As a practice, all important data are stored in HDDs and are copied to either floppy disk, E (E) PROM, CD as and when needed. COMPACT DISCS (CD) – These storage devices are of non-magnetic type and work with laser beam. Though there are many variants available, for understanding purpose we can consider what is known as Data disks, which can store up to 720 Megabytes of information. Other versions include Video CD (VCD), Digital Versatile Disk (DVD) which store up to 2 Giga bytes of data including songs, pictures and movies. One normal data CD is equivalent to 500 floppy disks or 375,000 pages of pure text!! PEN DRIVES – This product is the latest in storage devices list and as the name says it is very compact and is as small as cap of a pen. It can store up to 4 Giga bytes of information and is becoming very popular. Since pen drives are very delicate, they will not be used to store critical information but serves the purpose of an ordinary user. Broadly speaking, memory devices listed above are of permanent type, which means once some data is stored, it cannot be erased unless specific commands are given. This is something similar to writing on paper, which will remain permanently unless erased with a “rubber”. Apart from this, computers and controllers also use something called as temporary memory which will store the information only when needed. Once power is removed, all the contents will be lost. They are commonly called as RANDON ACCESS MEMORY (RAM). In future articles, we

will discuss more about their properties, applications and limitations. All memory devices are very sensitive to static electricity and storage conditions. They should be stored in cool place away from direct light and dust. They should not be stored close to current carrying conductors, high-frequency equipments like drives. Note: Next generation of storage devices, Blue Ray-discs can hold an astonishing 27 Giga bytes, equivalent to 1350000 (13.5 million) pages of text!!!

If you like to improvise this article or contribute or comment please mail us at: [email protected] This document contains information for reference only. We assume no responsibility for its implication.