What is PCI chips, PCIe Chips, PCI slots, types, and its functions

👤 Diwas Poudel    🕒 19 May 2022    📁 TECH

The motherboard is an important component of a computer. It is made up of a variety of components, including PCI slots and the chip. PCI is one of the Motherboard's useful components.

It is used in devices including modems, TV cards, memory, hard drive controllers, I/O ports, sound cards, graphics cards, USB, Coprocessors, and network cards.

Evolution of PCI and PCIe

Expansion slots like PCI have around 5 decades-long histories. Altair 8800 was the first slot-type expansion card bus added to a microcomputer in 1974-1975 which is located on the back panel of the PC. Later IBM develops ISA on IBM PC XT of 4.77 MB/second. Later in 1984, 16 bit ISA was developed with a bandwidth of 8 MB/second. Then MCA slots were developed but due to licensing all pc manufacturers cannot use them. Then came EISA but its speed cannot extend more than 8 MB/s. Then came VLB but due to compatibility with 486-class processors only cannot extend further and cannot supports Pentium processor.

The first industrial standard expansion slot named PCI appeared in 1992 by Intel. Many companies joined the alliance which is known today as PCI-SIG(PCI Special Interest Group). The standardization of the PCI, PCI-X, and PCI Express slots is the responsibility of the PCI-SIG.


PCI stands for Peripheral components interconnect. From this full form, it is clear that external hardware components are interconnected and make it work with other components by coordinating each other. When we talk about PCI then it includes both PCI slots on the motherboard and PCI chips themselves.

It's smart idea to pick wider case having a sufficient number of PCI expansion slots when buying computer. So  If you want to connect some cards later, such as wireless card, hard disk controller, USB slots, or TV tuner,  this will surely be helpful.

PCI bus Architecture

The PCI bus architecture is a processor-independent bus that allows peripherals to directly access the main memory without using the CPU. As a result, this helps to free up the CPU for other tasks.PCI simply takes the help of a bridge chip to access the system. Here, the bridge acts as a translation of transactions from the CPU bus to the PCI bus.

fig. PCI and PCIe slots(source)

Here we can see that PCI and PCIe buses are not directly connected to the CPU.PCI Slots are connected to Southbridge via PCI Bus, similarly, PCIe Slots are connected to northbridge via PCIe Bus. This shows that PCI Chips inserted into PCI Slots are indirectly connected and handle by the CPU.

Functions of PCI and PCI Express 

The function of PCI and PCI Express is used to connect additional external cards to the motherboard. There are different types of cards available to use in PCI slots some of them are Sound Cards, NIC Card, USB and Hard Drive Controller, Graphics Cards, RAID Controllers, etc. Suppose your ethernet card is not working then you can add an ethernet expansion card in PCI slots and enjoy ethernet card functionality.

Types of PCI Slots and PCI Chips

PCI slots are divided into 3 categories ie. PCI, PCI-X, and PCIe. They are divided on the basis of two main factors.

a) Bus Width: 32 bit or 64 bit

b) Signal Voltage : 3.3 V or 5 V

Different types of PCI slots are listed below:

  • PCI Slot
  • PCI-X 66
  • PCI-X 133
  • PCI-X 266
  • PCI-X 533
  • PCIe 1.0 x 1  , PCIe 1.0 x 4 , PCIe 1.0 x 8 , PCIe 1.0 x 16  
  • PCIe 2.0 x 1 ,PCIe 2.0 x 4 , PCIe 3.0 x 8 , PCIe 2.0 x 16  
  • PCIe 3.0 x 1 ,PCIe 3.0 x 4 , PCIe 3.0 x 8 , PCIe 3.0 x 16 
  • PCIe 4.0 x 1 ,PCIe 3.0 x 4 , PCIe 4.0 x 8 , PCIe 4.0 x 16 
  • PCIe 5.0 x 1 ,PCIe 3.0 x 4 , PCIe 5.0 x 8 , PCIe 5.0 x 16 
  • PCIe 6.0 x 1 ,PCIe 3.0 x 4 , PCIe 6.0 x 8 , PCIe 6.0 x 16 

PCIe is an extended version of PCI called PCI-Express.

 1) Conventional PCI

1. a. PCI Slot with 5 V signalling

This is the first version of PCI which operates at a 33 MHz bus clock using a 32-bit wide path and its bandwidth is 133 MB/s. Its signal voltage is 5 V.

Then 64-bit operating on 66 MHz with bandwidth = 133 MB/s was also developed operating on 5 V.

1 b. PCI Slot with 3.3 V signalling

Both 32-bit(33 MHz, 133MB/s) and 64-bit version(66 MHz,133 MB/s) of 3.3 V was developed.

💡 The PCI slot is also known as Conventional Slot.

At that time there is a problem and the problem is the PCI slot with 5V does not fit in the 3.3 V slot and vice versa. So, a universal PCI slot was developed.

1 c. Universal (3.3 V and 5 V) 32 bit PCI card 

This supports both 3.3 V and 5 V of 32 bit PCI Card. So, even if a 3.3 V 32 bit PCI Card is inserted into a Universal 32 bit slot then still it will work

1 d. Universal (3.3 V and 5V) 64 bit PCI card

This supports both 3.3 V and 5V of 64-bit PCI cards. So, if a 3.3 V 64 bit PCI card is inserted into a Universal 64 bit slot then still it will work.

💡  3.3V only PCI  cannot be plugged into a PC's 5V-only PCI.

Most of the PCI available in the market are 32 bit PCI.

So, the maximum speed of the PCI slot (3.3 v or 5 v) cannot exist at 133 MB/s and this didn't meet the requirements of modern PCs.So, PCI-SIG developed a new standard called PCI-X.

 2) PCI-X

PCI-X stands for PCI-eXtended. The number after X denotes indicates the operational frequency of PCI.

a. PCI-X 66

This version of PCI operates at 66 MHz bus clocks using a 64-bit wide path and its bandwidth is 533 MB/s. Here data per clock cycle is 1.

b. PCI-X 133

This version of PCI operates at 133 MHz bus clocks using a 64-bit wide path and its bandwidth is 1,066 MB/s. Here data per clock cycle is 1.

c. PCI-X 266

This version of PCI operates at 133 MHz bus clocks using a 64-bit wide path and its bandwidth is 2,132 MB/s. Here data per clock cycle is 2.

d. PCI-X 533

This version of PCI operates at 133 MHz bus clocks using a 64-bit wide path and its bandwidth is 4.266 MB/s. Here data per clock cycle is 4.

3) PCI-Express (PCIe)

fig. PCI Express Types

This version of PCI is constantly replacing PCI as the default expansion slot for add-in cards.

PCIe Types Clocks Wide Path Bandwidth
PCIe 1.0 x 1 2.5 GHz 1 bit 250 MB/s
PCIe 1.0 x 4 2.5 GHz 4 bit 1000 MB/s
PCIe 1.0 x 8 2.5 GHz 8 bit 2000 MB/s
PCIe 1.0 x 16 2.5 GHz 16 bit 4000 MB/s
PCIe 2.0 x 1 5 GHZ 1 bit 500 MB/s
PCIe 2.0 x 4 5 GHz 4 bit 2000 MB/s
PCIe 2.0 x 8 5 GHz 8 bit 4000 MB/s
PCIe 2.0 x 16 5 GHz 16 bit 8000 MB/s
PCIe 3.0 x 1 8 GHz 1 bit 1000 MB/s
PCIe 3.0 x 4 8 GHz 4 bit 4000 MB/s
PCIe 3.0 x 8 8 GHz 8 bit 8000 MB/s
PCIe 3.0 x 16 8 GHz 16 bit 16000 MB/s

Data Transfer in PCI 

1) Conventional PCI and PCI-X

In older PCI like Conventional PCI and PCI-X data are transferred in a parallel mode where several bits of data are transferred at the same time.

2) PCI-Express 

In the latest version of PCI like PCIe, data transfer is in a serial mode where a single bit of data is transferred at the same time. 

As we know that the parallel mode of data transfer is faster than the serial mode, then why the latest PCI adopted serial mode?🤔🤔🤔🤔🤔. If the same questions are raised in your mind then here is an answer.

➤ The modern motherboard is operating its task at a high frequency so, to get maximum benefits of the motherboard, a high-frequency PCI slot like PCIe is used. And PCIe uses a serial mode of data transfer. If data transfer in PCIe is the parallel mode then at high frequency large electromagnetic interference occurs which induces an electric current in an adjacent wire and induces current oppose the data transfer and data loss may occur.

Suppose, there are 32-bit PCIe slots and data transfer 32 bits at a time(parallelly) in 32 wires so, if higher the clock more interference occurs and data transmission loss may occur.  

Propagation delay is also a problem. Every bit transmits on a different wire in a parallel mode of communication, but it's nearly impossible to make the 32 wires on a motherboard exactly the same length. At a higher frequency, data transmitted by shorter wire arrive earlier than from longer wire to the receiving devices. So, receiving devices have to wait for all bits of data to arrive in order and this increased delay in the system and will be even worse if data is transferred at a high frequency.

So, in most of the high-frequency components like PCIe, serial communication is used. The negligible delay occurs but will guarantee data transfer.

Also, serial communication is preferred because, in serial, full-duplex(one set of wires to transmit data and another set of wires to receive data) communication is possible so sending and receiving devices can communicate at the same time but in parallel only half-duplex(same wires/path for receiving and sending data) is possible so in parallel two devices cannot communicate at the same time. Therefore, serial communication is preferred.

At lower frequencies, serial communication seems to be faster so, Conventional PCI uses serial Communication. At a higher frequency, parallel communication is preferred.


Do PCI cards work in PCIe Slots?

Ans: No. Because of the different configurations of PCI and PCIe.they are not compatible with each other.

What are the devices that use PCI Slots?

Ans:  PCI/e slots can be used to connect a wide range of electrical chips, including sound and graphics cards, network cards and modems as well as additional USB ports, HDD adapters and RAID controller cards.

What is PCIe cross-compatibility?

Ans: Because of PCIe cross-compatibility, a PCIe x1 card can be used in both x16 and x4 and x8 slots. The same is true for x4 cards, which can be installed in x8 or x16 slots.