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VESA LocalBus (VLB) (Technical)

This section is currently based solely on the work by Mark Sokos.

This file is intended to provide a basic functional overview of the Vesa Local Bus, so that hobbyists and amateurs can design their own VLB compatible cards.

It is not intended to provide complete coverage of the VLB standard.

VLB Connectors are usually inline with ISA connectors, so that adapter cards may use both. However, the VLB is separate, and does not need to connect to the ISA portion of the bus.

The 64 bit expansion of the bus (optional) does not add additional pins or connectors. Instead, it multiplexes the existing pins. The 32 bit VLB bus does not use the 64 bit signals shown in the above pinouts.

Signal Descriptions


Address Bus


Address Strobe


Byte Enable. Indicates that the 8 data lines corresponding to each signal will deliver valid data.


Burst Last. Indicates a VLB Burst Cycle, which will complete with *BRDY. The VLB Burst cycle consists of an address phase followed by four data phases.


Burst Ready. Indicates the end of the current burst transfer.


Data Bus. Valid bytes are indicated by *BE(x) signals.


Data/Command. Used with M/IO and W/R to indicate the type of cycle.

0 0 0 INTA sequence
0 0 1 Halt/Special (486)
0 1 0 I/O Read
0 1 1 I/O Write
1 0 0 Instruction Fetch
1 0 1 Halt/Shutdown (386)
1 1 0 Memory Read
1 1 1 Memory Write


Identification Signals.

ID0 ID1 ID4 CPU Bus Width Burst
0 0 0 (res)    
0 0 1 (res)    
0 1 0 486 16/32 Burst Possible
0 1 1 486 16/32 Read Burst
1 0 0 386 16/32 None
1 0 1 386 16/32 None
1 1 0 (res)    
1 1 1 486 16/32/64 Read/Write Burst


ID2 Indicates wait: 0 = 1 wait cycle (min)
  1 = no wait
ID3 Indicates bus speed: 0 = greater than 33.3 MHz
  1 = less than 33.3 MHz


Interrupt Request. Connected to IRQ9 on ISA bus. This allows standalone VLB adapters (not connected to ISA portion of the bus) to have one IRQ.


Local Enable Address Strobe. Set low by VLB master (not CPU). Also used for cache invalidation signal.


Local Bus Size 16. Used by slave device to indicate that it has a transfer width of only 16 bits.


Local Clock. Runs at the same frequency as the cpu, up to 50 MHz. 66 MHz is allowed for on-board devices.


Local Device: When appropriate address and M/IO signals are present on the bus, the VLB device must pull this line low to indicate that it is a VLB device. The VLB controller will then use the VLB bus for the transfer.


Local Ready. Indicates that the VLB device has completed the cycle. This signal is only used for single cycle transfers. *BRDY is used for burst transfers.


Local Grant. Indicates that an *LREQ signal has been granted, and control is being transferred to the new VLB master.


Local Request. Used by VLB Master to gain control of the bus.


Memory/IO. See D/C for signal description.


Ready Return. Indicates VLB cycle has been completed. May precede LRDY by one cycle.


Reset. Resets all VLB devices.


Write Back.

64-bit Expansion Signals


Acknowledge 64 bit transfer. Indicates that the device can perform the requested 64 bit transfer cycle.


Byte Enable. Indicates which bytes are valid (similar to BE0-BE3).


Upper 32 bits of data bus. Multiplexed with address bus.


Local Bus Size 64 bits. Used by VLB Master to indicate that it desires a 64 bit transfer.


Write/Read. See D/C for signal description.

64 Bit Data Transfer Timing Diagram:

             Address         Data
             Phase           Phase
             _______         _______         _______
LCLK     ___|       |_______|       |_______|       |_______

         ____         ______________________________________
*ADS         |_______|

              _______________  _______________
A2-A31   ----<_______________><_______________>-------------
D34-D63         Address          Data D34-D63

              _______________  _______________
D/C      ----<_______________><_______________>-------------
M/IO, W/R      M/IO, W/R         Data D32-33

         _____                 _____________________________
*LDEV         |_______________|

         _____                 _____________________________
*LBS64        |_______________|

         ______                _____________________________
*ACK64         |______________|

D0-D31    --------------------<_______________>-------------

          _____________________                _____________
LRDY                           |______________|
Contributor: Joakim Ögren, Mark Sokos
Sources: Mark Sokos VLB page
Sources: "The Indispensible PC Hardware Book" by Hans-Peter Messmer, ISBN 0-201-8769-3
Please send any comments to Joakim Ögren.