Computer buses and slots connectors pinouts  > obsolete pinouts
VLB=VESA Local Bus. VESA=Video Electronics Standards Association.

The VESA Local Bus is a local bus defined by the Video Electronics Standards Association, mostly used in personal computers based on the Intel 80486 CPU. VESA Local Bus worked alongside the ISA bus; it acted as a high-speed conduit for memory-mapped I/O and DMA, while the ISA bus handled interrupts and port-mapped I/O.

The VESA Local Bus was designed as a stop-gap solution to the problem of the ISA buss limited bandwidth. VESA had several flaws that served to limit its useful life substantially: 80486 dependence. The VESA Local Bus relied heavily on the 80486s memory bus design. When the Pentium processor started to gain mass acceptance, circa 1995, there were major differences in its bus design, and the VESA bus was not easily adaptable. This also made moving the bus to non-Intel architectures nearly impossible. Few Pentium motherboards with VESA slots were ever made. Limited number of slots available. Most PCs that used VESA Local Bus had only one or two slots available, as opposed to 5 or 6 ISA slots. This was because, as a direct branch of the 80486 memory bus, the VESA Local Bus didnt have the electrical ability to drive more than 1 or 2 (or 3 at the most) cards at a time.

Despite these problems, the VESA Local Bus was very commonplace on 486 motherboards. Probably a majority of 486-based systems had a VESA Local Bus video card, although early 486 systems never had VESA slots, as VESA debuted years after the introduction of the 486 processor.

Pin Name Description
A1 D1 Data 1
A2 D3 Data 3
A3 GND Ground
A4 D5 Data 5
A5 D7 Data 7
A6 D9 Data 9
A7 D11 Data 11
A8 D13 Data 13
A9 D15 Data 15
A10 GND Ground
A11 D17 Data 17
A12 Vcc +5 VDC
A13 D19 Data 19
A14 D21 Data 21
A15 D23 Data 23
A16 D25 Data 25
A17 GND Ground
A18 D27 Data 27
A19 D29 Data 29
A20 D31 Data 31
A21 A30 Address 30
A22 A28 Address 28
A23 A26 Address 26
A24 GND Ground
A25 A24 Address 24
A26 A22 Address 22
A27 VCC +5 VDC
A28 A20 Address 20
A29 A18 Address 18
A30 A16 Address 16
A31 A14 Address 14
A32 A12 Address 12
A33 A10 Address 10
A34 A8 Address 8
A35 GND Ground
A36 A6 Address 6
A37 A4 Address 4
A38 WBACK# Write Back
A39 BE0# Byte Enable 0
A40 VCC +5 VDC
A41 BE1# Byte Enable 1
A42 BE2# Byte Enable 2
A43 GND Ground
A44 BE3# Byte Enable 3
A45 ADS# Address Strobe
     
A48 LRDY# Local Ready
A49 LDEV Local Device
A50 LREQ Local Request
A51 GND Ground
A52 LGNT Local Grant
A53 VCC +5 VDC
A54 ID2 Identification 2
A55 ID3 Identification 3
A56 ID4 Identification 4
A57 LKEN#  
A58 LEADS# Local Enable Address Strobe
     
B1 D0 Data 0
B2 D2 Data 2
B3 D4 Data 4
B4 D6 Data 6
B5 D8 Data 8
B6 GND Ground
B7 D10 Data 10
B8 D12 Data 12
B9 VCC +5 VDC
B10 D14 Data 14
B11 D16 Data 16
B12 D18 Data 18
B13 D20 Data 20
B14 GND Ground
B15 D22 Data 22
B16 D24 Data 24
B17 D26 Data 26
B18 D28 Data 28
B19 D30 Data 30
B20 VCC +5 VDC
B21 A31 Address 31
B22 GND Ground
B23 A29 Address 29
B24 A27 Address 27
B25 A25 Address 25
B26 A23 Address 23
B27 A21 Address 21
B28 A19 Address 19
B29 GND Ground
B30 A17 Address 17
B31 A15 Address 15
B32 VCC +5 VDC
B33 A13 Address 13
B34 A11 Address 11
B35 A9 Address 9
B36 A7 Address 7
B37 A5 Address 5
B38 GND Ground
B39 A3 Address 3
B40 A2 Address 2
B41 n/c Not connected
B42 RESET# Reset
B43 DC# Data/Command
B44 M/IO# Memory/IO
B45 W/R# Write/Read
     
B48 RDYRTN# Ready Return
B49 GND Ground
B50 IRQ9 Interrupt 9
B51 BRDY# Burst Ready
B52 BLAST# Burst Last
B53 ID0 Identification 0
B54 ID1 Identification 1
B55 GND Ground
B56 LCLK Local Clock
B57 VCC +5 VDC
B58 LBS16# Local Bus Size 16

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

A2-A31

Address Bus

ADS

Address Strobe

BE0-BE3

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

BLAST

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.

BRDY

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

D0-D31

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

D/C

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

M/IO D/C W/R  
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

ID0-ID4

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

IRQ9

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.

LEADS

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

LBS16

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

LCLK

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

LDEV

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.

LRDY

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.

LGNT

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

LREQ

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

M/IO

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

RDYRTN

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

RESET

Reset. Resets all VLB devices.

WBACK

Write Back.

64-bit Expansion Signals

ACK64

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

BE4-BE7

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

D32-D63

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

LBS64

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

W/R

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                           |______________|
58 pin EDGE male connector layout
58 pin EDGE male connector
at the card
58 pin EDGE female connector layout
58 pin EDGE female connector
at the computer
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Source(s) of this and additional information: comp.sys.ibm.pc.hardware.* FAQ Part 4, maintained by Ralph Valentino, wikipedia.org, from Hardware Book, comp.sys.ibm.pc.hardware.* FAQ Part 4
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