---
title: "ROM and BJT memory cell storing 1 or 0"
subject: "Digital Logic"
chapter: 4
key: "semester_3/digital_logic:rom-bjt-memory-cell"
source: https://www.bctengineer.com/q/semester_3/digital_logic/rom-bjt-memory-cell
site: BCT Engineering
---

# ROM and BJT memory cell storing 1 or 0

Digital Logic · Chapter 4: Data Processing Circuits

- Asked in: 2080 Baisakh, 2+4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/digital_logic/rom-bjt-memory-cell
- Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-4

## Question

What is a ROM? Explain it how one bit memory is stored as '1' or '0', based on BJT circuit.

## Short answer

ROM is non-volatile memory with fixed data. In a BJT ROM cell, the transistor base connects to the word line, collector to Vcc, and emitter to the bit line if storing 1. When the row is selected, a 1 cell conducts pulling the column high; a 0 cell has no emitter link, so column stays low.

## Answer

**ROM (Read Only Memory)** is a non-volatile semiconductor memory in which data is written once (during manufacture or by programming) and afterwards can only be read. Data stays even when power is removed. It is used to store fixed programs (BIOS, firmware), look-up tables and code converters. Types: mask ROM, PROM, EPROM, EEPROM.

#### Storing 1 or 0 in a BJT ROM cell

A bipolar ROM is an array of **row (word) lines** and **column (bit) lines**. At each crossing there is a cell position where a BJT may or may not be connected.

- The **base** of the transistor is connected to the row line.
- The **collector** is connected to $V_{CC}$.
- The **emitter** is connected to the column line **only if a 1 is to be stored**. Each column line has a resistor to ground.

```text
 Vcc        Row line (from decoder)
  |          |
  C          |
   \|--------+       (stored '1')
   /| B
  E
  |
==+===== Column line --> sense --> Data = 1
         |
        [R]
         |
        GND

 Stored '0': emitter link absent (or fuse blown),
 so column stays at 0 V through R.
```

**Reading:**
1. The address decoder makes the selected row line HIGH.
2. In a cell storing **1**, the transistor's base is high, so it conducts (emitter follower). Current flows through the column resistor and the column line goes HIGH, which is read as **1**.
3. In a cell storing **0**, there is no emitter connection, so no current flows into that column; the resistor keeps it at 0 V, read as **0**.
4. Unselected rows are LOW, so their transistors are off and do not affect the columns.

In a mask ROM the connection is made or left out by the metal mask. In a bipolar PROM every cell is made with a fusible link in the emitter; the user stores a 0 by blowing the fuse with a high current pulse, and the intact fuse keeps a 1. Since the stored pattern is fixed by a physical connection, the data is non-volatile.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What characterises a ROM?
   A. Volatile memory that is read and written
   B. Memory that needs refreshing
   C. Memory that loses data on power off
   D. Non-volatile memory written once and then only read
   Answer: D. ROM retains data without power. (Key point: ROM definition)

2. In a BJT ROM, how is a 1 stored?
   A. The emitter is connected to the column line
   B. The base is left open
   C. The collector is grounded
   D. The emitter is removed
   Answer: A. A connected emitter lets the transistor drive the column high. (Key point: Storing 1)

3. When the selected row is HIGH, why does the column read 1?
   A. The resistor sources current from Vcc
   B. The transistor conducts as an emitter follower and drives current through the column resistor
   C. The row line connects to ground
   D. The transistor is turned off
   Answer: B. Current through the column resistor raises the column voltage. (Key point: Reading a 1)

4. In a bipolar PROM, how is 0 stored?
   A. The fuse is left intact
   B. UV light erases the cell
   C. The emitter fuse is blown by a high current pulse
   D. A mask omits the base
   Answer: C. An intact fuse keeps 1; blowing gives 0. (Key point: Storing 0 in PROM)

## Related in Digital Logic

- [RAM vs ROM; EEPROM cell](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Aram-rom-eeprom) — Digital Logic, chapter 4
- [Circuit giving 2's complement of a 4-bit number](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Atwos-complement-generator) — Digital Logic, chapter 5
- [2-bit equality comparator](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Aequality-comparator-2bit) — Digital Logic, chapter 4, asked 2×
- [4-bit equality comparator](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Aequality-comparator-4bit) — Digital Logic, chapter 4, asked 2×
- [2-bit magnitude comparator](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Amagnitude-comparator-2bit) — Digital Logic, chapter 4, asked 6×
- [3-bit magnitude comparator](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Amagnitude-comparator-3bit) — Digital Logic, chapter 4, asked 6×

## Related in other subjects

- [One Time Programmable ROM operation](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aotp-rom-storing) — Embedded System, chapter 4, asked 2×
- [BJT as a switch in power circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Abjt-as-power-switch) — Power Electronics, chapter 1, asked 2×
- [Astable multivibrator circuit (555 or BJT)](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Aamv-circuit-draw) — Electronics Devices and Circuit, chapter 5
- [Microcontroller-based thyristor firing circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Amicrocontroller-firing-opto) — Power Electronics, chapter 1