Computer Busses

by Peter Bjørn Perlsø

created 12th of March, 2005 , updated 22nd of June 2007 and 30aug2009.

This article briefly explains the historical background and evolution of the Personal Computer databus systems.

bus width (bits) clock theoretical peak bandwidth (MBps) burst capable year comments  
Industry Standard Architecture (ISA) - 8 bit 8 4.77 MHz 4.77   1981 used in PC XT  
ISA - 8 bit 8 8.33 MHz 8.33        
ISA - 16 bit 16 8.33 MHz 16.67   1984 used in PC AT  
Micro Channel Architecture (MCA) 16, 32 10.33 MHz up to 41.32   1987

Used exclusively in IBM's PS/2 systems - was IBM's attempt to marginalize Compaq - failed miserably because it was incompatible with the commonly used ISA.

Auto-configurable (Plug-n-play!), provides bus mastering, asynchronous

 
EISA 32 8.33 MHz 33 yes 1988

Launched by the "Gang of Nine" consortium of computer makers (AST, Compaq, Epson, Hewlett-Packard, NEC, Olivetti, Tandy, Wyse and Zenith) as an alternative to IBM's attempt to freeze out PC clone makers by way of MCA.

Auto-configurable (Plug-n-play!), provides bus mastering and divided interrupts.

 
NuBus 32 10 MHz 40 yes 1980's

"New Bus" - primarily used in Apple Macintoshes

Auto-configurable (Plug-n-play!)

 
               
VESA Local Bus (VLB) 32 up to 40 MHz 100, 133 or 160   1992 Used with 486 boards. Ran syncronously with the CPU, ie. at 25, 33 or 40 MHz  
Peripheral Component Interconnect (PCI) 32 33 MHz 132 yes 1992

developed by Intel, for use with Pentium systems. Asynchronous.

Auto-configurable (Plug-n-play!)

 
PCI 2.1 64 66 MHz 528 yes      
               
Accellerated Graphics Port (AGP) 32 66 MHz 264   Spring 1997 AGP 1.0, allows sideband addressing (demultiplexed (seperated) addresses from data -> faster data transfers) and AGP texturing ("direct execute mode" - system memory can be used as texture storage)  
2x AGP 32 2x66 MHz 528     "  
4x AGP 32 4x66 MHz 1056     AGP 2.0, allows write combining  
8x AGP 32 8x66 MHz 2112     AGP 3.0  
               
PCI-X 100 64 100 MHz 792 yes      
PCI-X 133 64 133 MHz 1056 yes      
PCI-X (2.0) 266 64 266 MHz 2.15 GBps yes   2003  
PCI-X (2.0) 533 64 533 MHz 4.3 GBps yes   2003  
               
PCI Express (PCIe) x16 2 ? up to 4 GBps yes   250 MBps per lane, up to 16 lanes (x16), more or less mainstream in late 2006  
               
               


Storage Bus Systems

 

bus width (bits) clock peak bandwidth (MBps) burst capable year comments  
AT Attachment (ATA), IDE 8 4,77 MHz 8,3   1981 used in PC AT (as the name suggests)  
Fast ATA (ATA-2), EIDE 8   16,7        
ATAPI 8            
ATA-3 16   16,7     includes ATAPI  
ATA-4 16   33     UltraDMA33  
ATA-5 16   66     80-conductor cables  
ATA-6 16   100        
ATA-7 16   133     >127 GB drive capacity  
               
Serial ATA (SATA) 1 1.5 GHz 150        
SATA-2 1 3 GHz 300     2005?  
SATA 6G (SATA-3) 1 6 GHz 600     2007??  
               
SCSI-1 8 5 MHz 5        
Fast SCSI (SCSI-2) 8 10 MHz 10        
Fast Wide SCSI (Wide SCSI-2) 16 10 MHz 20        
Ultra SCSI (SCSI-3) 8 20 MHz 20        
Ultra Wide SCSI (Wide SCSI-3) 16 20 MHz 40        
Ultra-2 SCSI 8 40 MHz 40     uses LVD cabling and connectors  
Ultra-2 Wide SCSI 16 40 MHz 80     "  
Ultra160 SCSI 16 80 MHz 160     ", (Ultra 3 Wide)  
Ultra320 SCSI 16 160 MHz 320     ", (Ultra 4 Wide)  
               
               


NB!

The term "Ultra Fast SCSI" is misleading and should be avoided. Ultra SCSI is actually an "extra Fast" SCSI, hence don't use the "Fast" term in conjunction with any Ultra SCSI name. Avoid misleading terms such as "Ultra Fast SCSI" and "Ultra Fast SCSI-3"

Ultra-2 SCSI is sometimes mistakenly referred to as "Ultra SCSI-2". Don't make that mistake.

Low Voltage Differential (LVD) SCSI and Differental SCSI are NOT the same things! Mixing up two of these bus/cabling types can have catastrophic consequences due to different signalling tensions and methods being used - beware!

Comments:

ALL Wide SCSI variants use 68-pin connectors, and they all allow for 16 storage units per channel.

SCSI info based on my old article "SCSI Demystified! (or how I learned to rid myself of the confusion and love SCSI)", based on a draft from 1998.

"How can they do that? THEY'RE CABLES, MAN!"
- Hudson, Aliens

:)

cc, 1998-2009, Peter Bjørn Perlsø