1 SI Units and Memory Sizes

2 SI Units: IEEE adopted the International System of Units in 1965 proposed in France 1960 (Systeme International d’Unites) Quantity SI Base Unit Unit Symbol length/distance meter m mass kilogram kg time second sec electric current ampere A

3

4 There are four commonly used units of measurement to describe memory capacity: kilobyte - kB megabyte- MB gigabyte- GB terabyte- TB

5 kB, One kilobyte -abbreviated k, kB, or k-byte - is equivalent to approximately 1000 bytes. Precisely, 1 kB = = 1024 bytes = 2 10 bytes. The older IBM PCs had a top capacity of 640KB, or about 640,000 characters of data. kB

6 MB, One megabyte - MB or M-byte - represents about 1 million (10 6 ) bytes. Precisely, 1 MB = = 1 kB x 1 kB = 1024 x 1024 = 2 10 x 2 10 = 2 20 = 1,048,576 bytes. MB

7 GB, One gigabyte - GB or G-byte - represents about 1 billion (10 9 ) bytes. Precisely, 1 GB = = 1 kB x 1 kB x 1 kB = 1024 x 1024 x 1024 = 2 10 x 2 10 x 2 10 = 2 30 = 1,073,741,824 bytes. GB

8 TB, One terabyte - TB or T-byte - represents 1 about trillion (10 12 ) bytes. Precisely, 1 TB = = 1 kB x 1 kB x 1 kB x 1 kB = 1024 x 1024 x 1024 x 1024 = 2 10 x 2 10 x 2 10 x 2 10 = 2 40 = 1,099,511,627,776 bytes. TB

9 An extra bit added to a byte for the purposes of error checking. –Even Parity:At the transmitter end the total number of “1”s is set to EVEN either by setting the parity bit (the ninth bit) to a ‘1’ or a ‘0’. –Odd Parity - At the transmitter end the total number of “1”s is set to ODD either by setting the parity bit (the ninth bit) to a ‘1’ or a ‘0’. Parity bit:

10

11 Error Checking –At the receiver end if the number of 1s in case of even parity is not even then there is an error occurred during the transmission. Error correction –To correct the error the receiver simply requests the transmitter to retransmit.

12 Safety Protect Yourself: –From Electricity:  Never ground yourself –  Do not disassemble/assemble a PC in bare feet on a cement or conductive floor.  Make sure the grounding strap has an internal resistor connected to it.  Finally, make sure the power cord is disconnected before you start working.  Assume a bare wire is live (110 volt) if you do not know the over all situation.

13 From Mechanical Pressure:  Never lift a monitor, system unit or a printer (which can be heavy) by bending your back.  Lift by bending your knees.  If possible, ask for assistance.  Work carefully inside a system unit, as it has sharp metal ends.

14 Protect the User:  If the computer system has metal body or metal parts exposed, make sure those are grounded when connected to the power outlet.  Check it with an Ohmmeter by removing the power cord from the wall socket. The ground pin should be connected to the exposed metal parts.

15 Protect the Hardware: –Static Electricity:  CMOS ICs are vulnerable to static electricity (200v to couple of thousands volt).  Work safely by discharging static electricity the following way:  Use grounding strap  Use grounding mats – A grounding mat is usually made of specially coated (conductive but has high resistance) rubber or plastic to maintain charge equal to ground. The mat should be connected to the ground just like a grounding strap.  If you don’t have any of the above items, make sure you discharged yourself by touching grounded materials.

16 Always keep the circuit boards in an Anti-static bag:  Use static shielded bags (ESD bags) – A ESD (ElectroStatic Discharge) bag is made of silver coated material (conductive but has high resistance) and bears an industry-recognized marking.  Keep these on a grounding mat or an anti-static bag.

17 Electricity:  Never short-circuit motherboards or interface cards.  Never drop a screw on a motherboard or an interface card, as they normally have a battery on it.  Never keep a motherboard or an interface card on a metal surface, as they normally have a battery on it.  Use surge protectors, to save the system from power spikes.

18 Protect the Software: –Rescue disk/System disk:  Keep a rescue disk. The purpose of keeping a rescue disk is to enable you to boot the system, in case the operating system or the hard drive fails. –Backups:  Backups should be done on systems regularly. How often data should be backed up depends on the value of the data. If the data is business critical then it should be backed up daily.  Some of the more common types of backup media include DAT tapes, ZIP tapes, CD-ROMs, JAZZ disks and other hard drives.

19 Documentation:  Store all documentation in a safe place.  If you want to add new hardware to a system you will want to first verify that it is compatible with your current hardware, as well as check for necessary jumper or DIP switch changes. Written or Electronic Record of CMOS Setup:

20 Preventative Maintenance Cleaning Supplies: –standard cleaner designed for use on computers or electronic assemblies preferably not in a spray –contact cleaner contains a lubricant that eases the force of connecting and disconnecting cables and connectors

21 –Canned air use on equipment that is powered off –Small brush and lint-free swabs to wipe off electrical contacts, connectors, disk drive heads … –Vacuum cleaner

22 Components that require cleaning: –Keyboard and Mouse –Monitor –Printers –Circuit Boards –Floppy Drives –Hard Drives software –Optical Drives –Tape Drives –Power Supply –Monitors

23 Environmental and Safety Hazards –Heat –Humidity high or low –Water –Food and Drink –Magnetic Interference –Static Electricity –Smoke –Power Use surge protectors, UPS, SPS, line conditioners