Input, Output Devices and Number Systems

Look at a banking branch, an examination hall, or even your own classroom — every computer interaction works through two simple ideas: data goes in, data comes out. The devices that handle these journeys are the input and output devices. IBPS PO Computer Awareness sections love this topic because two or three marks in this section are practically guaranteed if you know the device list cold.

Definition: An input device is any piece of hardware that feeds data or instructions into the computer. The user gives, the computer receives.

Definition: An output device is any piece of hardware that takes data out of the computer and presents it to the user, in a form the human senses can perceive — sight, sound, or touch.

Definition: An I/O device (input-output device) is a piece of hardware that performs both roles — it can both receive data from the user and deliver data back. Storage devices like hard disks are the most common example.

Input devices — the entry doors

The simplest mental rule is the direction of information flow: if data moves from a human (or the outside world) into the computer, the device is an input device. The standard IBPS-relevant list:

• Keyboard — the most familiar text input device. Has alphabetic, numeric, function and control keys. Inside, each keypress generates a scan code that the OS translates.
• Mouse — a pointing device. Its movement on a surface is translated into cursor motion on the screen.
• Scanner — converts a physical document or image into a digital file. Banks use scanners to capture KYC documents.
• Joystick — a stick-based pointing device, mainly for gaming and simulators.
• Light pen — a pen-like device that detects the screen position it touches; used in design and engineering.
• Microphone — converts sound waves into digital audio. Essential for voice notes and voice commands.
• Webcam — captures live video and feeds it into the computer.
• Barcode reader — reads barcodes printed on products. Used at every supermarket checkout in India.
• MICR — Magnetic Ink Character Recognition — reads the special magnetic-ink characters at the bottom of a bank cheque. The 9-digit MICR code identifies the bank, branch and city. Banks rely on it for cheque clearing; this is the single most exam-relevant device.
• OMR — Optical Mark Reader — reads filled bubbles on answer sheets. The OMR sheet you fill in the IBPS Prelims exam itself is read by an OMR machine.
• OCR — Optical Character Recognition — reads printed or handwritten characters from a scanned image and converts them into editable text. Used in digitising old records.

A useful sub-grouping: keyboard / mouse / joystick / light pen are pointing or typing devices; scanner / camera / microphone are capture devices; MICR / OMR / OCR / barcode are recognition devices — they convert a printed pattern into structured data.

Output devices — the exit doors

If the data moves from the computer to the user, the device is an output device.

• Monitor (also called VDU — Visual Display Unit) — displays text and graphics. Modern monitors are LED or LCD; older ones were CRT.
• Printer — produces hardcopy. Comes in several flavours discussed below.
• Speaker — converts digital audio back into sound waves.
• Projector — projects the screen onto a wall or screen, used in classrooms and presentations.
• Plotter — draws line drawings, used in engineering and architecture (e.g., printing building plans).

A working slogan: if it gives information to you, it is an output device.

I/O devices — both directions

Some devices do both jobs.

• Touchscreen — accepts your touch as input and displays output. Every modern smartphone is the perfect example.
• Modem — modulates outgoing digital signals into analog (output) and demodulates incoming analog signals into digital (input).
• Network card / NIC — receives and transmits data over a network.
• Headset (headphone + microphone) — input via mic, output via speakers.
• Hard disk / SSD — receives data when you save (input to the disk) and delivers data when you read (output from the disk). Often classified as storage I/O.

Printers — the most-tested sub-topic

The classification of printers is a near-certain exam question.

Impact printers physically strike an inked ribbon against paper.

• Dot-matrix printer — a print head with a grid of pins forms each character by striking the ribbon. Noisy, slow, low-resolution, but cheap and durable. Still used in railway reservation counters and many Indian Government offices for triplicate forms — because the impact lets the ink go through carbon copies.

Non-impact printers form characters without striking the paper.

• Inkjet printer — sprays tiny droplets of ink onto paper. Quiet, colour-capable, moderate speed, common in homes.
• Laser printer — uses a laser beam to draw the page on a rotating drum, then transfers toner (a fine powder) to paper and fuses it with heat. Fastest, highest quality, prints a whole page at a time. Found in offices and bank branches.

Memorise this table — it shows up as a one-mark MCQ almost every year.

Feature	Dot-matrix (impact)	Inkjet	Laser
Mechanism	Pins strike ribbon	Sprays ink	Toner + laser + heat
Speed	Slow	Medium	Fast
Quality	Low	Good	Best
Noise	High	Low	Low
Cost per page	Very low	Medium	Low (in volume)
Banking use	Triplicate forms	Rare	Statements, cheque books

Why it matters for IBPS PO

Banking sections specifically test MICR, OMR, OCR, barcode readers, and printer types because banks themselves rely on these devices every day. A clerk who does not know what MICR encodes will not survive the first month at the counter.

Real-world example

Walk into any SBI branch in India and you can see almost every device in action — a keyboard and mouse at every desk, a scanner for KYC docs, an MICR reader in the cheque-clearing room, a barcode reader at the cash counter for noting batches, dot-matrix printers at the passbook-printing station (because they have to push ink through multi-layer continuous stationery), and laser printers at the manager's desk for statements. The branch is essentially a textbook diagram come alive.

Common misconception

Common misconception: Many students think MICR, OMR, and OCR are interchangeable. They are not.

• MICR uses magnetic ink — it can read characters even if dirty or stamped over. Only on bank cheques.
• OMR reads positions of marks (filled bubbles), not characters. Used on answer sheets.
• OCR reads shapes of characters — recognises actual letters and digits from a scanned image.

If the question says "cheque," the answer is MICR. If it says "exam answer sheet," the answer is OMR. If it says "scanned text," the answer is OCR.

Worked example

Question: Which of the following is NOT an output device — (a) Monitor, (b) Plotter, (c) MICR, (d) Speaker?

Solution:
Step 1: Monitor displays output. Output device. Tick.
Step 2: Plotter draws diagrams. Output device. Tick.
Step 3: MICR reads magnetic-ink characters from cheques and feeds them in. Input device.
Step 4: Speaker produces sound output. Output device. Tick.

Conclusion: The correct answer is (c) MICR. This is the exact phrasing IBPS uses in past papers — recognising the trap takes you straight to the mark.

Computers use four number systems:

• Binary (base 2): digits 0-1 — the machine's native language.
• Octal (base 8): digits 0-7.
• Decimal (base 10): digits 0-9 — human everyday numbers.
• Hexadecimal (base 16): digits 0-9 and A-F (A=10, B=11, C=12, D=13, E=14, F=15).

CONVERSIONS:

• Decimal to Binary: divide by 2 repeatedly, read remainders bottom-up.
• Binary to Decimal: multiply each bit by its place value (powers of 2) and sum.
• Binary↔Octal: group bits in 3s. Binary↔Hex: group bits in 4s.

Memory aid: Hex 'A through F' = 10 through 15. 1 hex digit = 4 bits (a nibble); 1 octal digit = 3 bits. This grouping trick makes conversions instant.

When you stand on a bridge above the Yamuna in Delhi and notice the water foaming and the fish missing, you are looking at a single biological number gone wrong — the Biochemical Oxygen Demand, or BOD. It is the most-tested water-pollution indicator in NEET, and grasping it deeply is worth at least one direct mark every year.

Definition: Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen (in mg) that would be consumed by aerobic microorganisms in one litre of water while they decompose all the organic matter present in that water. It is measured over a fixed period (commonly five days at 20°C — hence BOD$_5$).

What BOD really measures

BOD is not a direct measure of "how dirty" water looks. It is a measure of how much biodegradable organic matter is in the water, expressed indirectly through the oxygen that bacteria will eat while breaking that matter down.

Picture a glass of clean stream water and a glass of sewage placed side by side, both sealed with bacteria inside. The sewage glass has plenty of organic food (proteins, sugars, fats from human and animal waste). Bacteria multiply rapidly, respiring aerobically, and consume vast amounts of oxygen. The clean water glass has very little organic food; bacteria barely metabolise, and oxygen is largely untouched.

The DIFFERENCE in oxygen levels — initial minus final — is the BOD.

This is why higher BOD means more organic pollution and more microbial activity, and lower BOD means cleaner water. The number is a stand-in for the invisible weight of organic muck dissolved in the water.

The link to fish kills

Dissolved oxygen is what fish, prawns, and most aquatic invertebrates breathe with their gills. The atmosphere contains 21% oxygen; water, even saturated, holds only about 8 mg per litre of dissolved oxygen at room temperature. That tiny budget is what every aquatic organism shares.

When sewage enters a river, the BOD shoots up — bacterial respiration drains the oxygen budget. Fish suffocate. Sensitive species die first, then hardier ones. The water turns black and smelly because anaerobic bacteria take over and release hydrogen sulphide (H$_2$S, the rotten-egg smell). What looked like pollution from a chemistry perspective is, biologically, death by oxygen starvation.

Memory aid: BOD high → Oxygen down → fish die.

Why it matters

In NEET, BOD is the standard indicator examiners use to test understanding of water pollution. It appears in:

• Direct assertion-reason questions ("BOD measures organic pollution; high BOD means clean water" — the second half is the trap).
• Comparison questions where you must rank water samples by BOD against habitat health.
• Conceptual questions linking BOD to eutrophication and to the death of aquatic life.

The NCERT Class 12 textbook devotes nearly a full page to BOD in the Environmental Issues chapter, which is a strong signal of its NEET frequency. Master this one concept and you usually pick up 1 direct mark plus assist questions on water pollution.

Real-world example

The Ganga at Varanasi typically has BOD values of 4-8 mg/L upstream, but spikes to 25-40 mg/L downstream of the city — well above the safe limit of 3 mg/L set by the Central Pollution Control Board (CPCB) for designated bathing waters. Domestic sewage and dye effluents drive the spike. The Yamuna near Okhla, infamously, has recorded BOD as high as 60-70 mg/L during dry summer months, which is why the river smells, foams, and supports almost no fish in that stretch. The Namami Gange and Yamuna Action Plan are essentially BOD-reduction missions, even when public messaging calls them "river cleaning."

Common misconception

"High BOD means high oxygen in the water." Exactly the reverse. High BOD means high oxygen DEMAND — bacteria are GOING to consume a lot of oxygen, so the water has very LOW dissolved oxygen available for fish. NEET sets this trap by phrasing it as "BOD high means oxygen content is high" — and the careless student says True.

A second misconception: "BOD measures all pollution." No. BOD only measures biodegradable organic pollution — sewage, food waste, animal manure, biodegradable industrial effluent. Heavy metals, pesticides, and non-biodegradable chemicals are not captured by BOD; they need separate indicators (COD — Chemical Oxygen Demand — or specific toxicity tests).

A third trap: "Higher BOD always means more bacteria are alive." During the active decomposition phase, yes. But if BOD stays high while oxygen runs out, aerobic bacteria die and anaerobic bacteria take over — the bacterial community shifts, not just its size.

BOD vs COD — the partner test

Definition: Chemical Oxygen Demand (COD) is the oxygen needed to chemically oxidise BOTH biodegradable AND non-biodegradable organic matter in water, using a strong chemical oxidant. COD is usually higher than BOD because it includes what bacteria can't digest. The ratio BOD/COD tells you what fraction of the pollution is biologically breakdownable — useful for designing sewage treatment plants.

A worked example

Question: Three water samples have BOD values of 2 mg/L, 18 mg/L, and 0.5 mg/L respectively. Which sample is most likely to support a healthy fish population, and which is most polluted with organic waste?

Solution:
Step 1: Recall — low BOD means low organic pollution and more dissolved oxygen for fish.
Step 2: Sample with 0.5 mg/L has the lowest BOD — most oxygen available, healthiest for fish.
Step 3: Sample with 18 mg/L has the highest BOD — most organic pollution, worst for aquatic life.
Step 4: Cross-check against the CPCB safe BOD limit of 3 mg/L for bathing-quality water — samples 1 and 3 are within limits; sample 2 is severely polluted.
Conclusion: Healthiest = 0.5 mg/L sample; most polluted = 18 mg/L sample.

How BOD connects to other NEET concepts

• Eutrophication. Nutrient runoff (nitrogen and phosphorus from fertilisers and sewage) triggers algal blooms. When the algae die, bacteria decompose them — driving up BOD and crashing oxygen levels. BOD is the bridge between nutrient pollution and fish kills.
• Sewage treatment. Primary treatment removes solids; secondary treatment uses aerobic bacteria to LOWER the BOD before discharge. The whole engineering goal of a sewage plant is to lower BOD below the legal limit.
• Indicator species. Mayfly larvae and stoneflies need low BOD; sludge worms thrive in high BOD. Biologists actually count these species to assess BOD without instruments — a clever NEET-style cross-link.

This topic delivers two easy question types in IBPS PO: (1) classify a device as input/output, and (2) number-system bases and conversions. Banks love MICR (cheque processing) and OMR/OCR — know them exactly.

Input devices (feed data IN)

Keyboard, mouse, joystick, light pen, touchscreen, scanner, webcam, microphone, and the special readers:

• OMR (Optical Mark Reader): reads pencil marks — used in OMR answer sheets / objective exams.
• OCR (Optical Character Reader): reads printed/handwritten characters.
• MICR (Magnetic Ink Character Reader): reads magnetic-ink numbers at the bottom of bank cheques — high-yield for bankers.
• Barcode/QR reader, Biometric scanner.

Output devices (give results OUT)

Monitor (VDU), printer, plotter, speaker, projector, headphones.

• Impact printers: strike the paper — Dot Matrix, Line printer.
• Non-impact printers: Inkjet (sprays ink), Laser (toner + drum, fastest for offices).
• Plotter: for large engineering/architectural drawings.
  Devices that are BOTH input & output: touchscreen, modem, network card, headset, hard disk.

Number systems

System	Base	Digits
Binary	2	0,1
Octal	8	0–7
Decimal	10	0–9
Hexadecimal	16	0–9, A–F

Computers work internally in binary. In hex, A=10, B=11, C=12, D=13, E=14, F=15.
Conversion basics: to convert decimal→binary, divide by 2 and read remainders bottom-up. Binary→decimal: multiply each bit by its place value (…8,4,2,1) and add. Example: 1011₂ = 8+0+2+1 = 11₁₀. Group binary in 4s for hex, in 3s for octal.

Exam Tricks & Tips

• 🎯 MICR = cheques (bankers' favourite). OMR = marks/OMR sheets. OCR = characters. Mnemonic: MICR–Money, OMR–marks, OCR–characters.
• 🎯 Scanner, keyboard, mouse, mic = input; monitor, printer, speaker = output.
• 🎯 Laser & Inkjet = non-impact; Dot-matrix = impact.
• 🎯 Binary place values from right: 1,2,4,8,16,32,64,128. Learn them for fast conversion.
• 🎯 Hex letters: A=10 … F=15; a hex digit = exactly 4 binary bits.
• ❌ Common mistake: calling plotter an input device — it is OUTPUT (draws large graphics).

Expected exam pattern

"Which is an input device?", "MICR is used for?", "Base of hexadecimal is?", "Convert 1010₂ to decimal?", "Which printer is impact?". Quick recall + tiny conversion.

Quick recap

Input: keyboard/mouse/scanner/OMR/OCR/MICR. Output: monitor/printer/plotter/speaker. Both: touchscreen/modem. Bases: binary 2, octal 8, decimal 10, hex 16. Hex A–F = 10–15. MICR = cheques.

Cover the answer, recall, then check. 12 cards on I/O devices and number systems.

Q1. What does MICR read, and where is it used?
A1. Magnetic Ink Character Reader — reads magnetic-ink numbers on bank cheques.

Q2. Differentiate OMR and OCR.
A2. OMR (Optical Mark Reader) reads pencil marks (OMR sheets); OCR (Optical Character Reader) reads printed/handwritten characters.

Q3. Give three pure input devices.
A3. Keyboard, mouse, scanner, joystick, microphone, light pen (any three).

Q4. Give three pure output devices.
A4. Monitor, printer, plotter, speaker, projector (any three).

Q5. Name two devices that are both input and output.
A5. Touchscreen, modem, network card, headset, hard disk (any two).

Q6. Impact vs non-impact printer — give one example each.
A6. Impact: dot-matrix (strikes paper). Non-impact: laser or inkjet.

Q7. State the base and digits of the binary, octal and hexadecimal systems.
A7. Binary: base 2 (0,1); Octal: base 8 (0–7); Hexadecimal: base 16 (0–9, A–F).

Q8. In hexadecimal, what decimal values do A and F represent?
A8. A = 10 and F = 15 (B=11, C=12, D=13, E=14).

Q9. Convert binary 1011 to decimal.
A9. 8 + 0 + 2 + 1 = 11.

Q10. How many binary bits equal one hexadecimal digit?
A10. 4 bits (one hex digit = a nibble).

Q11. Which number system does a computer use internally?
A11. Binary (base 2) — only 0s and 1s.

Q12. Is a plotter an input or output device? What is it used for?
A12. Output device — used to print large, high-quality engineering/architectural drawings.