1.
THE EXTERNAL BUS

2.
INTRODUCTION
• What Is a Bus ?
• The External Bus
• Early IBM PC Had Only 256 KB of Memory
• The Expansion Bus - a Collection of Expansion Slots
• Four Major Industry Standards
1.PC Bus
2.ISA Bus
3. EISA Bus
4. MCA Bus
• How Many Expansion Slots ? (Is More Good or Bad)
• Multi function expansion boards
• Multi function mother boards

3.
THE PC BUS
• The First IBM PC Was Based on
» 8088 Microprocessor (8-bit)
» 4.77 MHz
• The 62-pin Printed Edge Card Connector Had
» 6 Interrupt Lines
» 20 Address Lines
» 1 Mbyte Addressable memory

4.
ISA BUS
• Industry Standard Architecture
• IBM Introduced 286-AT
• Specifications
• 16-bit Data Handling Capability
• 24-bit Address Lines
• Backward Compatible
• ISA Bus Added the IRQs and DMA Control Lines
• Very Cheap and Easy to Manufacture
• Limitations
• Only 4 Fast & Non-latchable Address Lines Available
• Could Only Map the Nearest 64 KB of Memory
• Special Software Not Available for Signaling Procedure
• Undefined by IBM so No Standardization
• Worked Only at a Fixed 8 MHz Speed

5.
THE NEED FOR CHANGE
• The need for change because of 386
– Only 8 MHz
– 24-bit address lines
– 16-bit data lines
• The RAM bus
• Disadvantage : The loss of standardization
• Introduction of SIPP and SIMM modules

6.
The EISA Bus
• Basically bi-level modified ISA Edge Connectors
• Backward compatible with ISA & PC Bus
• EISA Specifications
1. 32-bit bus
2. 8 MHz
3. Sharable interrupt lines (level sensitive)
4. Bus mastering facilities
5. Data Transfer Rate (Max) = 32 MB/S
6. Auto detection of add on boards
(Configuration file needed)

7.
The MCA Bus
• Complete incompatible with ISA & EISA
• Designed for IBM PS/2 machines
• Available in 16-bit & 32-bit versions
• Video extension may be added
• A synchronous bus (Fixed clock speed)
• Data Transfer Rate = 160 MB/S
• Supports multiple bus masters
• Shareable Interrupt lines (level sensitive)
• Easy configuration using PO/S

8.
CHOOSING THE BUS
ISA EISA MCA
Cost Low High Higher
Performance Low --”-- --”--
Space High High Low
Required
Backward Yes Yes No
Compatibility
Max Data 6.5 MB/S 25 MB/S 160 MB/S
Transfer Rate
Bus saturation
“Data rate demand exceeds the data rate that the bus can handle”.