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PCI Local Bus Technical Summary

Table of Contents

1.0 PCI Overview

2.0 PCI Documents

3.0 PCI Bus Protocol

4.0 PCI Signal Descriptions

5.0 PCI Bus Timing Diagrams

6.0 PCI Connector Pinout


1.0 PCI Overview

The PCI Local Bus is a high performance bus for interconnecting chips, expansion boards, and processor/memory subsystems. It originated at Intel in the early 1990s as a standard method of interconnecting chips on a board. It was later adopted as an industry standard administered by the PCI Special Interest Group, or "PCI SIG". Under the PCI SIG the definition of PCI was extended to define a standard expansion bus interface connector for add-in boards.

PCI was first adopted for use in personal computers in about 1994 with Intel's introduction of the "Saturn" chipset and "Alfredo" motherboard for the 486 processor. With introduction of chipsets and motherboards for the Intel Pentium processor, PCI largely replaced earlier bus architectures such as EISA, VL, and Micro Channel. The ISA bus has initially continued to co-exist with PCI for support of "legacy" add-in boards that don't require the high performance of the PCI bus. But as legacy boards are redesigned, PCI is expected to completely replace ISA as well.

On September 11, 1998 the PCI SIG announced that Compaq, Hewlett-Packard, and IBM had submitted a new specification for review called "PCI-X". The proposed standard allows for increases in PCI bus speed up to 133 MHz. It also includes suggested changes in the PCI communications protocol affecting data transfer rates and electrical timing requirements. The PCI-SIG has approved the formation of a working group to review the proposal.


2.0 PCI Documents

2.1 PCI Specifications

Copies of the PCI Local Bus Specifications may be ordered for a fee from the PCI SIG. The following is the release history of the PCI specification:

The PCI SIG also maintains the following PCI related documents:

2.2 PCI Books

Two recommended books on PCI are:


3.0 PCI Bus Protocol

PCI is a synchronous bus architecture with all data transfers being performed relative to a system clock (CLK). The initial PCI specification permitted a maximum clock rate of 33 MHz allowing one bus transfer to be performed every 30 nanoseconds. Later, Revision 2.1 of the PCI specification extended the bus definition to support operation at 66 MHz, but the vast majority of today's personal computers continue to implement a PCI bus that runs at a maximum speed of 33 MHz.

PCI implements a 32-bit multiplexed Address and Data bus (AD[31:0]). It architects a means of supporting a 64-bit data bus through a longer connector slot, but most of today's personal computers support only 32-bit data transfers through the base 32-bit PCI connector. At 33 MHz, a 32-bit slot supports a maximum data transfer rate of 132 MBytes/sec, and a 64-bit slot supports 264 MBytes/sec.

The multiplexed Address and Data bus allows a reduced pin count on the PCI connector that enables lower cost and smaller package size for PCI components. Typical 32-bit PCI add-in boards use only about 50 signals pins on the PCI connector of which 32 are the multiplexed Address and Data bus. PCI bus cycles are initiated by driving an address onto the AD[31:0] signals during the first clock edge called the address phase. The address phase is signaled by the activation of the FRAME# signal. The next clock edge begins the first of one or more data phases in which data is transferred over the AD[31:0] signals.

In PCI terminology, data is transferred between an initiator which is the bus master, and a target which is the bus slave. The initiator drives the C/BE[3:0]# signals during the address phase to signal the type of transfer (memory read, memory write, I/O read, I/O write, etc.). During data phases the C/BE[3:0]# signals serve as byte enable to indicate which data bytes are valid. Both the initiator and target may insert wait states into the data transfer by deasserting the IRDY# and TRDY# signals. Valid data transfers occur on each clock edge in which both IRDY# and TRDY# are asserted.

A PCI bus transfer consists of one address phase and any number of data phases. I/O operations that access registers within PCI targets typically have only a single data phase. Memory transfers that move blocks of data consist of multiple data phases that read or write multiple consecutive memory locations. Both the initiator and target may terminate a bus transfer sequence at any time. The initiator signals completion of the bus transfer by deasserting the FRAME# signal during the last data phase. A target may terminate a bus transfer by asserting the STOP# signal. When the initiator detects an active STOP# signal, it must terminate the current bus transfer and re-arbitrate for the bus before continuing. If STOP# is asserted without any data phases completing, the target has issued a retry. If STOP# is asserted after one or more data phases have successfully completed, the target has issued a disconnect.

Initiators arbitrate for ownership of the bus by asserting a REQ# signal to a central arbiter. The arbiter grants ownership of the bus by asserting the GNT# signal. REQ# and GNT# are unique on a per slot basis allowing the arbiter to implement a bus fairness algorithm. Arbitration in PCI is "hidden" in the sense that it does not consume clock cycles. The current initiator's bus transfers are overlapped with the arbitration process that determines the next owner of the bus.

PCI supports a rigorous auto configuration mechanism. Each PCI device includes a set of configuration registers that allow identification of the type of device (SCSI, video, Ethernet, etc.) and the company that produced it. Other registers allow configuration of the device's I/O addresses, memory addresses, interrupt levels, etc.

Although it is not widely implemented, PCI supports 64-bit addressing. Unlike the 64-bit data bus option which requires a longer connector with an additional 32-bits of data signals, 64-bit addressing can be supported through the base 32-bit connector. Dual Address Cycles are issued in which the low order 32-bits of the address are driven onto the AD[31:0] signals during the first address phase, and the high order 32-bits of the address (if non-zero) are driven onto the AD[31:0] signals during a second address phase. The remainder of the transfer continues like a normal bus transfer.

PCI defines support for both 5 Volt and 3.3 Volt signaling levels. The PCI connector defines pin locations for both the 5 Volt and 3.3 Volt levels. However, most early PCI systems were 5 Volt only, and did not provide active power on the 3.3 Volt connector pins. Over time more use of the 3.3 Volt interface is expected, but add-in boards which must work in older legacy systems are restricted to using only the 5 Volt supply. A "keying" scheme is implemented in the PCI connectors to prevent inserting an add-in board into a system with incompatible supply voltage.

Although used most extensively in PC compatible systems, the PCI bus architecture is processor independent. PCI signal definitions are generic allowing the bus to be used in systems based on other processor families.

PCI includes strict specifications to ensure the signal quality required for operation at 33 and 66 MHz. Components and add-in boards must include unique bus drivers that are specifically designed for use in a PCI bus environment. Typical TTL devices used in previous bus implementations such as ISA and EISA are not compliant with the requirements of PCI. This restriction along with the high bus speed dictates that most PCI devices are implemented as custom ASICs.

The higher speed of PCI limits the number of expansion slots on a single bus to no more than 3 or 4, as compared to 6 or 7 for earlier bus architectures. To permit expansion buses with more than 3 or 4 slots, the PCI SIG has defined a PCI-to-PCI Bridge mechanism. PCI-to-PCI Bridges are ASICs that electrically isolate two PCI buses while allowing bus transfers to be forwarded from one bus to another. Each bridge device has a "primary" PCI bus and a "secondary" PCI bus. Multiple bridge devices may be cascaded to create a system with many PCI buses.


4.0 PCI Signal Descriptions

Required Pins                            Optional Pins
-------------                            -------------
                   ----------------
                  |                |
<===AD[31:0]=====>|                |<===AD[63:32]====>
<===C/BE[3:0]#===>|     PCI        |<===C/BE[7:4]#===>
<---PAR---------->|   Compliant    |<---PAR64-------->
                  |    Device      |<---REQ64#------->
<---FRAME#------->|                |<---ACK64#------->
<---TRDY#-------->|                |
<---IRDY#-------->|                |<---LOCK#-------->
<---STOP#-------->|                |
<---DEVSEL#------>|                |----INTA#-------->
----IDSEL-------->|                |----INTB#-------->
                  |                |----INTC#-------->
<---PERR#-------->|                |----INTD#-------->
<---SERR#-------->|                |
                  |                |<---SBO#--------->
<---REQ#----------|                |<---SDONE-------->
----GNT#--------->|                |
                  |                |<---TDI-----------
----CLK---------->|                |----TDO---------->
----RST#--------->|                |<---TCK-----------
                  |                |<---TMS-----------
                  |                |<---TRST#---------
                   ----------------

4.1 System Pins

CLK
Clock provides the timing reference for all transfers on the PCI bus. All PCI signals except reset and interrupts are sampled on the rising edge of the CLK signal. All bus timing specifications are defined relative to the rising edge. For most PCI systems the CLK signal operates at a maximum frequency of 33 MHz. Revision 2.1 of the PCI specification defined a 66 MHz operating mode, but this mode is not yet widely implemented. To operate at 66MHz, both the PCI system and the PCI add-in board must be specifically designed to support the higher CLK frequency. Add-in boards indicate to the system if they are 66 MHz capable through the M66EN signal. A 66 MHz system will supply a 66 MHz CLK if the add-in board supports it, and supply a default 33 MHz CLK if the add-in board does not support the higher frequency. Likewise, if a system is capable of providing only a 33 MHz clock, then a 66 MHz add-in board must be able to operate using the lower frequency. The minimum frequency of the CLK signal is specified at 0 Hz permitting CLK to be "suspended" for power saving purposes.
RST#
Reset is driven active low to cause a hardware reset of a PCI device. The reset shall cause a PCI device's configuration registers, state machines, and output signals to be placed in their initial state. RST# is asserted and deasserted asynchronously to the CLK signal. It will remain active for at least 100 microseconds after CLK becomes stable.

4.2 Address and Data Pins

AD[31:0]
Address and Data are multiplexed onto these pins. AD[31:0] transfers a 32-bit physical address during "address phases", and transfers 32-bits of data information during "data phases". An address phase occurs during the clock following a high to low transition on the FRAME# signal. A data phase occurs when both IRDY# and TRDY# are asserted low. During write transactions the initiator drives valid data on AD[31:0] during each cycle it drives IRDY# low. The target drives TRDY# low when it is able to accept the write data. When both IRDY# and TRDY# are low, the target captures the write data and the transaction is completed. For read transactions the opposite occurs. The target drives TRDY# low when valid data is driven on AD[31:0], and the initiator drives IRDY# low when it is able to accept the data. When both IRDY# and TRDY# are low, the initiator captures the data and the transaction is completed. Bit 31 is the most significant AD bit. Bit 0 is the least significant AD bit.
C/BE[3:0]#
Bus Command and Byte Enables are multiplexed onto these pins. During the address phase of a transaction these signals carry the bus command that defines the type of transfer to be performed. See the table below for a list of valid bus command codes. During the data phase of a transaction these signals carry byte enable information. C/BE[3]# is the byte enable for the most significant byte (AD[31:24]) and C/BE[0]# is the byte enable for the lease significant byte (AD[7:0]). The C/BE[3:0]# signals are driven only by the initiator and are actively driven through the all address and data phases of a transaction.
C/BE[3:0]#Command Types
0000Interrupt Acknowledge
0001Special Cycle
0010I/O Read
0011I/O Write
0100Reserved
0101Reserved
0110Memory Read
0111Memory Write
1000Reserved
1001Reserved
1010Configuration Read
1011Configuration Write
1100Memory Read Multiple
1101Dual Address Cycle
1110Memory Read Line
1111Memory Write and Invalidate
PAR
Parity is even parity over the AD[31:0] and C/BE[3:0]# signals. Even parity implies that there is an even number of '1's on the AD[31:0], C/BE[3:0]#, and PAR signals. The PAR signal has the same timings as the AD[31:0] signals, but is delayed by one cycle to allow more time to calculate valid parity.

4.3 Interface Control Pins

FRAME#
Cycle Frame is driven low by the initiator to signal the start of a new bus transaction. The address phase occurs during the first clock cycle after a high to low transition on the FRAME# signal. If the initiator intends to perform a transaction with only a single data phase, then it will return FRAME# back high after only one cycle. If multiple data phases are to be performed, the initiator will hold FRAME# low in all but the last data phase. The initiator signals its intent to perform a master initiated termination by driving FRAME# high during the last data phase of a transaction. During a target initiated termination the initiator will continue to drive FRAME# low through the end of the transaction.
IRDY#
Initiator Ready is driven low by the initiator as an indication it is ready to complete the current data phase of the transaction. During writes it indicates the initiator has placed valid data on AD[31:0]. During reads it indicates the initiator is ready to accept data on AD[31:0]. Once asserted, the initiator holds IRDY# low until TRDY# is driven low to complete the transfer, or the target uses the STOP# signal to terminate without performing the data transfer. IRDY# permits the initiator to insert wait states as needed to slow the data transfer.
TRDY#
Target Ready is driven low by the target as an indication it is read to complete the current data phase of the transaction. During writes it indicates the target is ready to accept data on AD[31:0]. During reads it indicates the target has placed valid data on the AD[31:0] signals. Once asserted, the target holds TRDY# low until IRDY# is driven low to complete the transfer. TRDY# permits the target to insert wait states as needed to slow the data transfer.
STOP#
Stop is driven low by the target to request the initiator terminate the current transaction. In the event that a target requires a long period of time to respond to a transaction, it may use the STOP# signal to suspend the transaction so the bus can be used to perform other transfers in the interim. When the target terminates a transaction without performing any data phases it is called a retry. If one or more data phases are completed before the target terminates the transaction, it is called a disconnect. A retry or disconnect signals the initiator that it must return at a later time to attempt performing the transaction again. In the event of a fatal error such as a hardware problem the target may use STOP# and DEVSEL# to signal an abnormal termination of the bus transfer called a target abort. The initiator can use the target abort to signal system software that a fatal error has been detected.
LOCK#
Lock may be asserted by an initiator to request exclusive access for performing multiple transactions with a target. It prevents other initiators from modifying the locked addresses until the agent initiating the lock can complete its transaction. Only a specific region (a minimum of 16 bytes) of the target's addresses are locked for exclusive access. While LOCK# is asserted, other non-exclusive transactions may proceed with addresses that are not currently locked. But any non-exclusive accesses to the target's locked address space will be denied via a retry operation. LOCK# is intended for use by bridge devices to prevent deadlocks.
IDSEL
Initialization Device Select is used as a chip select during during PCI configuration read and write transactions. IDSEL is driven by the PCI system and is unique on a per slot basis. This allows the PCI configuration mechanism to individually address each PCI device in the system. A PCI device is selected by a configuration cycle only if IDSEL is high, AD[1:0] are "00" (indicating a type 0 configuration cycle), and the command placed on the C/BE[3:0]# signals during the address phase is either a "configuration read" or "configuration write". AD[10:8] may be used to select one of up to eight "functions" within the PCI device. AD[7:2] select individual configuration registers within a device and function.
DEVSEL#
Device Select is driven active low by a PCI target when it detects its address on the PCI bus. DEVSEL# may be driven one, two, or three clocks following the address phase. DEVSEL# must be asserted with or prior to the clock edge in which the TRDY# signal is asserted. Once DEVSEL# has been asserted, it cannot be deasserted until the last data phase has completed, or the target issues a target abort. If the initiator never receives an active DEVSEL# it terminates the transaction in what is termed a master abort.

4.4 Arbitration Pins (Initiator Only)

REQ#
Request is used by a PCI device to request use of the bus. Each PCI device has its own unique REQ# signal. The arbiter in the PCI system receives the REQ# signals from each device. It is important that this signal be tri-stated while RST# is asserted to prevent a system hang. This signal is implemented only be devices capable of being an initiator.
GNT#
Grant indicates that a PCI device's request to use the bus has been granted. Each PCI device has its own unique GNT# signal from the PCI system arbiter. If a device's GNT# signal is active during one clock cycle, then the device may begin a transaction in the following clock cycle by asserting the FRAME# signal. This signal is implemented only be devices capable of being an initiator.

4.5 Error Reporting Pins

PERR#
Parity Error is used for reporting data parity errors during all PCI transactions except a "Special Cycle". PERR# is driven low two clock periods after the data phase with bad parity. It is driven low for a minimum of one clock period. PERR# is shared among all PCI devices and is driven with a tri-state driver. A pull-up resistor ensures the signal is sustained in an inactive state when no device is driving it. After being asserted low, PERR# must be driven high one clock before being tri-stated to restore the signal to its inactive state. This ensures the signal does not remain low in the following cycle because of a slow rise due to the pull-up.
SERR#
System Error is for reporting address parity errors, data parity errors during a Special Cycle, or any other fatal system error. SERR# is shared among all PCI devices and is driven only as an open drain signal (it is driven low or tri-stated by PCI devices, but never driven high). It is activated synchronously to CLK, but when released will float high asynchronously through a pull-up resistor.

4.6 Interrupt Pins

INTA#, INTB#, INTC#, INTD#
Interrupts are driven low by PCI devices to request attention from their device driver software. They are defined as "level sensitive" and are driven low as an open drain signal. Once asserted, the INTx# signals will continue to be asserted by the PCI device until the device driver software clears the pending request. A PCI device that contains only a single function shall use only INTA#. Multi-function devices (such as a combination LAN/modem add-in board) may use multiple INTx# lines. A single function device uses INTA#. A two function device uses INTA# and INTB#, etc. All PCI device drivers must be capable of sharing an interrupt level by chaining with other devices using the interrupt vector.

4.7 Cache Support Pins (Optional)

These pins are architected to permit cacheable memory to be implemented on a PCI bus. They transfer status information between the bridge/cache and the target of the memory request. If a PCI transaction results in a hit on a "dirty" cache line, the bridge/cache will signal "snoop backoff" to the cacheable target. As a result, the target will issue retries on all accesses to the modified cache line until the bridge/cache completes a writeback operation. The target will then permit the access to complete.

These cache support pins are rarely if ever implemented in today's PCI systems. For performance reasons, cacheable memory is typically coupled very closely with a host processor bus that runs at a higher frequency than PCI.

SBO#
Snoop Backoff indicates a hit to a modified line when asserted. When SBO# is deasserted and SDONE is asserted, it indicates a "CLEAN" snoop result.
SDONE
Snoop Done indicates the status of the snoop for the current access. When deasserted, it indicates the result of the snoop is still pending. When asserted, it indicates the snoop is complete.

4.8 Additional Pins

PRSNT[1:2]#
Present signals are used for two purposes: 1) to indicate that an add-in board is physically present, and 2) to indicate the power requirements of an add-in board. These are static signals that are either grounded or left open on the add-in board. Refer to the following table for the encoding of these signals.
PRSNT1#PRSNT2#Add-in Board Configuration
Open Open No board present
GroundOpen Board present, 25W maximum
Open GroundBoard present, 15W maximum
GroundGroundBoard present, 7.5W maximum
CLKRUN#
Clock Running is an optional signal used to facilitate stopping of the CLK signal for power saving purposes. CLKRUN# is intended only for the "mobile" environment where power consumption is critical. It is not defined on the PCI connector used for regular add-in boards. CLKRUN# is driven as an open drain signal. The PCI system drives CLKRUN# low when it is propagating a normal CLK signal. It releases CLKRUN# so it floats to a high level via a pull-up resistor as a request to stop the CLK for a specific PCI device. The device may then pulse CLKRUN# low to indicate to the system that it should continue to drive CLK, or allow CLKRUN# to remain high as confirmation that CLK can be stopped. If the CLK has been stopped and a PCI device wants to resume normal operation, it drives CLKRUN# low as a request that the system should start driving CLK again.
M66EN
66MHZ Enable is left "open" or disconnected on add-in boards that support operation with a 66 MHz CLK, and grounded on add-in boards that support operation with only a 33 MHz CLK. 66 MHz systems place a pull-up resistor on this signal to detect if the add-in board is 66 MHz capable. If the signal is high, a CLK with a maximum frequency of 66 MHz is supplied. If it is low, a CLK with a maximum frequency of 33 MHz is supplied. 33 MHz systems attach this signal to ground. 66 MHz operation will take place only if both the system and the add-in board support it.

4.9 64-Bit Bus Extension Pins (Optional)

AD[63:32]
Address and Data are multiplexed on the same pins and provide 32 additional bits when operating in a 64-bit bus environment. During data phases these bits transfer an additional 32-bits of data when both REQ64# and ACK64# are asserted. During address phases, when a Dual Address Cycle is being issued and the REQ64# signal is asserted, these bits transfer the upper 32-bits of the address.
C/BE[7:4]#
Bus Command and Byte Enables are multiplexed onto the same pins and provide 4 additional bits when operating in a 64-bit bus environment. During data phases these bits transfer byte enables for the upper 32-bits of the data bus (AD[63:32]) when both REQ64# and ACK64# are asserted. During address phases, when a Dual Address Cycle is being issued and the REQ64# signal is asserted, these bits transfer the bus command.
REQ64#
Request 64-bit Transfer is asserted low by the initiator to indicate it desires a 64-bit transfer. This signal is driven with the same timings as FRAME#.
ACK64#
Acknowledge 64-bit Transfer is asserted low by a target as an indication that it has decoded its address as the target of the current access, and is capable of performing a 64-bit transfer.
PAR64
Parity Upper DWORD is the even parity bit that protects AD[63:32] and C/BE[7:4]#.

4.10 JTAG/Boundary Scan Pins (Optional)

PCI devices may optionally support JTAG/Boundary Scan as defined in IEEE Standard 1149.1, Test Access Port and Boundary Scan Architecture. JTAG allows components installed on a PCI add-in board to be exhaustively tested by serially scanning test patterns through each component. The following signals are defined by the JTAG standard. If JTAG is not implemented by an add-in board, the TDI and TDO signals must be connected to preserve the scan path.

TCK
Test Clock
TDI
Test Data Input
TDO
Test Output
TMS
Test Mode Select
TRST#
Test Reset


5.0 PCI Bus Timing Diagrams

5.1 Read Transaction

Read Transaction

The following timing diagram illustrates a read transaction on the PCI bus:

            1__   2__   3__   4__   5__   6__   7__   8__   9__
CLK       __|  |__|  |__|  |__|  |__|  |__|  |__|  |__|  |__|
          ____                                     ____________
FRAME#        |___________________________________|
               _____       _____ _____  ____  __________
AD        ----<_____>-----<_____>_____><____><__________>------
              Address      Data1        Data2    Data3
               _____  __________________________________
C/BE#     ----<_____><__________________________________>------
              Bus-Cmd              BE#'s
          __________                         _____       ______
IRDY#               |_______________________|     |_____|
          ________________       _____                   ______
TRDY#                     |_____|     |_________________|
          ________________                               ______
DEVSEL#             |_____|_____________________________|

              |<--->|<--------->|<--------->|<--------->|
              Address   Data        Data        Data
              Phase     Phase       Phase       Phase
              |<--------------------------------------->|
                            Bus Transaction
The following is a cycle by cycle description of the read transaction:

5.2 Write Transaction

The following timing diagram illustrates a write transaction on the PCI bus:

            1__   2__   3__   4__   5__   6__   7__   8__   9__
CLK       __|  |__|  |__|  |__|  |__|  |__|  |__|  |__|  |__|
          ____                         ________________________
FRAME#        |_______________________|
               _____  ____  ____ _____  _____________
AD        ----<_____><____><____>_____><_____________>---------
              Address Data1 Data2           Data3
               _____  ____  ____  ______________________
C/BE#     ----<_____><____><____><______________________>------
              Bus-Cmd BE-1  BE-2            BE-3
          __________             _____                   ______
IRDY#               |___________|     |_________________|
          __________             _________________       ______
TRDY#               |___________|                 |_____|
          ________________                               ______
DEVSEL#             |_____|_____________________________|

              |<--->|<--->|<--->|<--------------------->|
              Address Data  Data        Data
              Phase   Phase Phase       Phase
              |<--------------------------------------->|
                            Bus Transaction
The following is a cycle by cycle description of the read transaction:


6.0 PCI Connector Pinout

The following table illustrates the pinout definition for the PCI connector. The PCI specification defines two types of connectors that may be implemented at the system board level: One for systems that implement 5 Volt signaling levels, and one for systems that implement 3.3 Volt signaling levels.

In addition, PCI systems may implement either the 32-bit or 64-bit connector. Most PCI buses implement only the 32-bit portion of the connector which consists of pins 1 through 62. Advanced systems which support 64-bit data transfers implement the full PCI bus connector which consists of pins 1 through 94.

Three types of add-in boards may be implemented: "5 Volt add-in boards" include a key notch in pin positions 50 and 51 to allow them to be plugged only into 5 Volt system connectors. "3.3 Volt add-in boards" include a key notch in pin positions 12 and 13 to allow them to be plugged only into 3.3 Volt system connectors. "Universal add-in boards" include both key notches to allow them to be plugged into either 5 Volt or 3.3 Volt system connectors. Universal boards must be able to adapt to operation at either signaling level.

Pin 5V System Environment Pin 3.3V System Environment Comments
Side BSide A Side BSide A
1 -12V TRST# 1 -12V TRST# 32-bit start
2 TCK +12V 2 TCK +12V
3 Ground TMS 3 Ground TMS
4 TDO TDI 4 TDO TDI
5 +5V +5V 5 +5V +5V
6 +5V INTA# 6 +5V INTA#
7 INTB# INTC# 7 INTB# INTC#
8 INTD# +5V 8 INTD# +5V
9 PRSNT1# Reserved9 PRSNT1# Reserved
10 Reserved +5V (I/O)10 Reserved +3.3V (I/O)
11 PRSNT2# Reserved11 PRSNT2# Reserved
12 Ground Ground 12 Connector Key3.3V key
13 Ground Ground 13 Connector Key3.3V key
14 Reserved Reserved14 Reserved Reserved
15 Ground RST# 15 Ground RST#
16 CLK +5V (I/O)16 CLK +3.3V (I/O)
17 Ground GNT# 17 Ground GNT#
18 REQ# Ground 18 REQ# Ground
19 +5V (I/O)Reserved19 +3.3V (I/O)Reserved
20 AD[31] AD[30] 20 AD[31] AD[30]
21 AD[29] +3.3V 21 AD[29] +3.3V
22 Ground AD[28] 22 Ground AD[28]
23 AD[27] AD[26] 23 AD[27] AD[26]
24 AD[25] Ground 24 AD[25] Ground
25 +3.3V AD[24] 25 +3.3V AD[24]
26 C/BE[3]# IDSEL 26 C/BE[3]# IDSEL
27 AD[23] +3.3V 27 AD[23] +3.3V
28 Ground AD[22] 28 Ground AD[22]
29 AD[21] AD[20] 29 AD[21] AD[20]
30 AD[19] Ground 30 AD[19] Ground
31 +3.3V AD[18] 31 +3.3V AD[18]
32 AD[17] AD[16] 32 AD[17] AD[16]
33 C/BE[2]# +3.3V 33 C/BE[2]# +3.3V
34 Ground FRAME# 34 Ground FRAME#
35 IRDY# Ground 35 IRDY# Ground
36 +3.3V TRDY# 36 +3.3V TRDY#
37 DEVSEL# Ground 37 DEVSEL# Ground
38 Ground STOP# 38 Ground STOP#
39 LOCK# 3.3V 39 LOCK# 3.3V
40 PERR# SDONE 40 PERR# SDONE
41 +3.3V SBO# 41 +3.3V SBO#
42 SERR# Ground 42 SERR# Ground
43 +3.3V PAR 43 +3.3V PAR
44 C/BE[1]# AD[15] 44 C/BE[1]# AD[15]
45 AD[14] +3.3V 45 AD[14] +3.3V
46 Ground AD[13] 46 Ground AD[13]
47 AD[12] AD[11] 47 AD[12] AD[11]
48 AD[10] Ground 48 AD[10] Ground
49 Ground AD[09] 49 M66EN AD[09]
50 Connector Key50 Ground Ground 5V key
51 Connector Key51 Ground Ground 5V key
52 AD[08] C/BE[0]#52 AD[08] C/BE[0]#
53 AD[07] +3.3V 53 AD[07] +3.3V
54 +3.3V AD[06] 54 +3.3V AD[06]
55 AD[05] AD[04] 55 AD[05] AD[04]
56 AD[03] Ground 56 AD[03] Ground
57 Ground AD[02] 57 Ground AD[02]
58 AD[01] AD[00] 58 AD[01] AD[00]
59 +5V (I/O)+5V (I/O)59 +3.3V (I/O)+3.3V (I/O)
60 ACK64# REQ64# 60 ACK64# REQ64#
61 +5V +5V 61 +5V +5V
62 +5V +5V 62 +5V +5V 32-bit end
Connector Key Connector Key64-bit spacer
Connector Key Connector Key64-bit spacer
63 Reserved Ground 63 Reserved Ground 64-bit start
64 Ground C/BE[7]#64 Ground C/BE[7]#
65 C/BE[6]# C/BE[5]#65 C/BE[6]# C/BE[5]#
66 C/BE[4]# +5V (I/O)66 C/BE[4]# +3.3V (I/O)
67 Ground PAR64 67 Ground PAR64
68 AD[63] AD[62] 68 AD[63] AD[62]
69 AD[61] Ground 69 AD[61] Ground
70 +5V (I/O)AD[60] 70 +3.3V (I/O)AD[60]
71 AD[59] AD[58] 71 AD[59] AD[58]
72 AD[57] Ground 72 AD[57] Ground
73 Ground AD[56] 73 Ground AD[56]
74 AD[55] AD[54] 74 AD[55] AD[54]
75 AD[53] +5V (I/O)75 AD[53] +3.3V (I/O)
76 Ground AD[52] 76 Ground AD[52]
77 AD[51] AD[50] 77 AD[51] AD[50]
78 AD[49] Ground 78 AD[49] Ground
79 +5V (I/O)AD[48] 79 +3.3V (I/O)AD[48]
80 AD[47] AD[46] 80 AD[47] AD[46]
81 AD[45] Ground 81 AD[45] Ground
82 Ground AD[44] 82 Ground AD[44]
83 AD[43] AD[42] 83 AD[43] AD[42]
84 AD[41] +5V (I/O)84 AD[41] +3.3V (I/O)
85 Ground AD[40] 85 Ground AD[40]
86 AD[39] AD[38] 86 AD[39] AD[38]
87 AD[37] Ground 87 AD[37] Ground
88 +5V (I/O)AD[36] 88 +3.3V (I/O)AD[36]
89 AD[35] AD[34] 89 AD[35] AD[34]
90 AD[33] Ground 90 AD[33] Ground
91 Ground AD[32] 91 Ground AD[32]
92 Reserved Reserved92 Reserved Reserved
93 Reserved Ground 93 Reserved Ground
94 Ground Reserved94 Ground Reserved64-bit end

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