---
title: "Short introduction to ISA bus"
subject: "Instrumentation II"
chapter: 2
key: "semester_5/instrumentation_ii:isa-bus"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/isa-bus
site: BCT Engineering
---

# Short introduction to ISA bus

Instrumentation II · Chapter 2: Parallel Interfacing With Microprocessor Based System

- Asked in: 2068 Chaitra, 3 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/isa-bus
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-2

## Question

Give a short introduction of ISA bus.

## Short answer

The ISA bus is the original PC expansion bus, with 8-bit (XT) or 16-bit (AT) data paths, up to 16 MB addressing, and 8 MB/s transfer rate. It uses manual configuration and supports DMA and interrupts, but is obsolete, replaced by PCI.

## Answer

The **ISA (Industry Standard Architecture) bus** is the expansion bus of the IBM PC/XT/AT, used to plug adapter cards (serial, parallel, sound, network, disk controllers) into the motherboard.

- **XT bus:** 8-bit data, 20 address lines (1 MB), 62-pin connector, 4.77 MHz.
- **AT (ISA) bus:** 16-bit data, 24 address lines (16 MB), added 36-pin connector, 8 MHz (8.33 MHz).
- Signals: address (SA0-SA19, LA17-LA23), data (SD0-SD15), control (MEMR', MEMW', IOR', IOW', BALE, AEN, I/O CH RDY), interrupts (IRQ2-IRQ15), DMA (DRQ, DACK', T/C), power (+5 V, -5 V, +12 V, -12 V).
- Data transfer rate is about 8 MB/s; manual configuration by jumpers; supports DMA and interrupts through the 8237 and 8259.
- Obsolete now; replaced by PCI because of its low speed and lack of plug and play.

## Check yourself

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

1. What is the ISA bus?
   A. A processor-to-memory bus only
   B. The expansion bus of the IBM PC/XT/AT for plug-in adapter cards
   C. A serial network standard
   D. A graphics-only bus
   Answer: B. ISA lets adapter cards such as serial, sound and disk controllers plug into the motherboard. (Key point: What ISA is)

2. How does the AT (ISA) bus differ from the XT bus?
   A. 16-bit data and 24 address lines (16 MB)
   B. 8-bit data and 20 address lines
   C. 32-bit data and 32 address lines
   D. 64-bit data at 66 MHz
   Answer: A. XT is 8-bit/20 lines; AT adds a 36-pin connector for 16-bit/24 lines. (Key point: AT bus)

3. Why is ISA obsolete?
   A. Low speed (about 8 MB/s) and no plug and play
   B. It is too fast for modern cards
   C. It needs no configuration
   D. It has too many data lines
   Answer: A. ISA uses jumpers and is slow, so PCI replaced it. (Key point: Weaknesses)

## Related in Instrumentation II

- [ISA bus signals; replacement by PCI bus](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aisa-pci-bus) — Instrumentation II, chapter 2

## Related in other subjects

- [ISA bus protocols](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aisa-bus-protocols) — Embedded System, chapter 5