Industry Standard Architecture Bus
[PC Bus Description(s)]
[ISA Card Form Factors] [PC
Interface ICs]
[PC Connectors] [PC-XT / PC-AT Pinout] [PCAT Cards] [PC Chassis]
[Home]
ISA (AT)bus: The ISA bus operates at 8MHz with an 8 and 16 bits
data bus, a 24 bit address bus, using +/- 12 volts, +/- 5 volts, and 15
Interrupt lines. The standard drive level is 24mA for all non
Open-Collector signals on the bus. The AT card used the standard (edge)
connector provided by the XT bus and added an additional (edge) connector
behind that with the same pin-spacing @ 0.1 inch center-to-center. The
additional connector has only 38 (19 per side) fingers, while the XT
connector had 62 (32 per side) fingers. The Motherboard could then accept either an
8 or 16 bit card in an 8 bit slot (XT), or (if the connector was
provided) a 16 bit card in an AT slot. A maximum number of 8 Expansion
slots were provided on IBM compatible Motherboards. The additional
connector provided 4 additional address lines , and 8 additional data
lines. Some Interrupt lines are reserved and the AT spec allowed
interrupt sharing. A number of different oscillators may be used to
control timers, bus transfers, etc. Refer to this page for a comparison
of Video bus through-put for
different expansion buses. This page provides the PC-XT / PC-AT
pinout.
By 2001, many computers started shipping with
PCI only slots. But a combination of ISA AT expansion slots, and
PCI slots was still common. The PC ISA bus is Obsolete and should not be
used for new designs. In fact, all of the buses listed here are obsolete,
and were all replaced by the PCI bus. But, because so many ISA cards and
Mother boards were produced
many companies still support these form factors. By the middle of 2004
many companies were shipping Mother Boards with PCI expansion slots
[which replaced the ISA slots] and PCI-Express slots, in a
combination of 1x PCIe slots and 16x PCIe slots, which replaced the
AGP expansion slots. As of 2005
PCI-express slots reside on a majority of motherboards, but AGP expansion
slots are still common [especially on AMD based boards]. PCI slots still
exist on all motherboards, and the ISA expansion slots are all but gone.
Only industrial PC motherboards still offer ISA based motherboards. In
addition; the Embedded Industrial PC/104 specification (IEEE P966) still
requires the ISA bus, in a different form factor.
Additional Personal Computer [PC] Bus Information:
ISA (XT)bus: Obsolete; 4.77MHz @ 8 bits, +/- 12 volts, +/- 5
volts. The XT bus used a 62 pin (.1" center) edge connector; 31 pins per
card side. Used a single oscillator of 14.31818MHz which was divided by
3. 8 Data lines, 0 to 7 (LSB=0). 20 Address lines, 0 to 19 (LSB=0). 1
Clock line (4.77MHz). 1 Reset line, 8 Interrupt lines. Some 8 bit cards
have skirts which extend the board below the depth of the top of the
connector to allow additional circuitry. These cards, with skirts, are
not compatible with the 16 bit AT bus. The XT bus uses connector J1
(A/B), AT uses J1 (A/B), J2 (C/D). A maximum number of 8 Expansion slots
were provided on IBM compatible Motherboards.
MCA (Micro Channel Architecture) bus: Obsolete; 10MHz @ 16 or 32
bits, uP independent, asynchronous, IBM proprietary on PS2 computers.
With bus enhancements the speed reaches 80MBps, using clock doubling. The
MCI bus only appeared on IBM PS2 series of computers which have been off
the market for many years now. MCA description and pinout information is
listed on the MCA page.
EISA (Extended Industry Standard Architecture) or (Enhanced ISA)
bus: 8MHz @ 8/16/32 bits data bus, 32 bit address bus; PC Expansion
Bus, compatible with ISA. An ISA card will work in a EISA slot, but an
EISA card will not work in an AT slot. All of the PC-XT and PC-AT fingers
reside on an EISA board/connector. The actual EISA fingers (pin) reside
below the XT and AT fingers on an EISA board. The EISA bus (in one mode)
used both edges of the clock, with the rising edge used to output
address, and the falling edge to place the data on the bus. Three other
transfer modes were available. The EISA bus does not allow the board
skirts common with the older XT cards. The EISA cards are the same size
as the AT cards. The new address lines are termed "LA#", all address
lines are latched. Refer to the EISA Pinout table for signal names
and pin out information.
PCI (Peripheral Component Interconnect) bus: The
Peripheral Component Interface 'PCI' [Parallel] Bus was originally
developed as a local bus expansion for the PC. The first version of the
PCI bus ran at 33MHz with a 32 bit bus (133MBps), the current version
runs at 66MHz with a 64 bit bus. The PCI bus operates either
synchronously or asynchronously with the "mother Board bus rate: The PCI
bus is microprocessor independent. The page contains the PCI connector
Pin-Outs. The PCI bus replaced the PCAT bus on most Mother Boards.
PISA (PCI + ISA) bus: ~PC Expansion Bus. A normal ISA card with an
additional row of pins above, used for the PCI bus. A passive backplane
which moves all active devices off the Mother-Board and onto a single
card. The controller card used in the system has fingers [edge
connectors] for both PCI and the ISA bus, the Mother Board only
connectors. This allows additional cards to be added to the mother board
which use either the ISA or PCI buses. Because only connectors reside on
the motherboard, repair time is increased, and down time is decreased.
The standard is PICMG-xx. The specification is used in embedded or
industrial computer systems.
VESA (Video Electronic Standards Association)/Local bus (VLB or VL-Bus): had a bus speed of
33MHz using either 16-bit or 32-bit data transfers, 30 address bits were
provided. A 64 bit bus was also available by multiplexing the additional
upper 32 bits on to address bus. A maximum of three devices may be
connected to the bus. Best speed is seen in the Burst mode: Single
address followed by 4 data transfers. Supports bus mastering.
The VLB resides on a standard 16 bit
ISA card with the additional pins required by the VLB interface residing
after the ISA pins, allowing an ISA card to use the same slot. A VLB card
does not need to use the ISA bus and may only require the VLB connector
pins. Rev 2 of VLB multiplexed 32 bit of data onto the address bus,
producing a 64 bit data bus. The version 2 card is back-ware compatible.
The VLB connector is keyed at pins 46 and 47. The pinout and a short
description is provided on the VESA Local Bus page.
Because this bus used the same bus structure as the 486, it could only be
used with the 486 class of processors, with the advent of the Pentium
processor and the PCI bus - this
bus is now obsolete. This bus was replaced with a deviant of the PCI bus
called the AGP bus.
Apple Computer Buses {Pin-Outs only}
All other Computer Interface Buses
PC AT and combined AT/PCI Bus Board Dimensions Full Size Detailed (with Pin-Outs)
PC PCI Board Dimensions Half Size Board with Detailed Dimensions
The PC-104 card is an industrial embedded card that uses the PCAT bus.
The PC-104 card also uses a different board format, or form factor as shown in the graphic.
ISA bus use standard TTL logic levels for the AT and XT PC
bus interfaces. An input Low is 0 to 0.4v, High is 2.4v to Vcc. An output
Low is 0 to 0.8v, High is 2.0v to Vcc.
The bus drive was reduced from
24mA used on the ISA bus to 4mA for the embedded PC/104 bus.
PC Chip Set Manufacturers
Memory Chip Manufacturers
Memory Module Manufacturers
Computer Processor Manufacturers
TTL Glue Logic Manufacturers
PCAT Connector: Card Edge type, 100 pins, dual row (50 pins per side)
on 0.1" centers.
The connector is keyed. For a comparison between the IBM
XT slot vs. the AT slot, refer to the connector below.
The IBM XT slot used the left portion of the connector (J1). The IBM AT slot added the
right most portion of the connector (J2).
The AT slot used the entire connector, J1 and J2. The connector has a card slot barrier which divides
the 98 Pin Connector into Groups of 62 and 36 Contacts.
EDAC Inc.
{PC/AT-EISA-MCA}
FCI
{Card Edge Connectors, 120, 184, 180 pins}
Card Edge connector manufacturers are listed on the connector page.
Note that other than the pitch or pin to pin spacing, the PC-AT connector is not much different than other PC expansion connectors.
Topic Navigation: Engineering Home > Interface Buses > Personal Computer Buses > PC-AT Expansion Slot.
Home | |||||||
Distributors | Components | Equipment | Software | Standards | Buses | Design | Reference |