ISA Bus / AT Bus

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]

PC/PC-AT Description and Pin Outs

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 The PC ISA bus is Obsolete and should not be used for new designs.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

{PCAT/ISA Bus Index}

Card [Dimensions] Form Factors

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

PC-AT Board Size Drawing

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.

{PCAT Bus Index}

PC Bus Interface IC Vendors

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

{PC-AT Bus Index}

PC Bus Connector 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.

PC Edge Connector Size
ISA Physical Connector


{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.

{ISA Bus Index}

PC motherboard

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Modified 6/13/15
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