The PCI Express bus (formerly known as 3GIO) represents an extraordinary advance in the way peripheral devices communicate with the computer. It differs from the PCI bus in many aspects, but the most important one is the way data is transferred. The PCI Express bus is an example of how PC data transfer is migrating from parallel communication to serial communication. Read our article Why Serial? to understand the differences between serial and parallel communications.

Almost all PC buses (ISA, EISA, MCA, VLB, PCI and AGP) use parallel communication. Parallel communication differs from the serial one because it transmits several bits at a time, while in serial communication only one bit is transmitted at a time. This makes, at first, parallel communication faster than the serial one, since the higher the number of bits transmitted at a time, the faster the communication will be.

But parallel communication suffers from some problems that prevent transmissions from reaching higher clocks. The higher the clock, the greater will be the problems with magnetic interference and propagation delay.

When the electric current passes flows through a wire, an electromagnetic field is created around it. If the electromagnetic field created by the wire happens to be very strong, noise will be produced in the near wire, corrupting the information being transmitted. As in parallel transmission several bits are transmitted at a time, each bit involved in the transmission uses one wire. For example, in a 32-bit communication (such as the PCI slot) it’s necessary to have 32 wires just to transmit data, not counting additional control signals that are also necessary. The higher the clock, the greater the electromagnetic interference problem.

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Figure 1: Bits arrive at the receptor that was corrupted due to electromagnetic interference problem.

As we have commented before, each bit in parallel communication is transmitted in a separate wire. But it’s almost impossible to make those 32 wires have exactly the same length in a motherboard. This difference in wire length didn’t alter the way the bus worked in older PCs, but due to the increased speed in which data is transmitted (clock), data transmitted through shorter wires started to arrive before the rest of the data that was transmitted through longer wires. That is, the bits in parallel communication started to arrive out of order.

As a consequence, the receptor device has to wait for all bits to arrive in order to process the complete data, which represents significant loss in performance. This problem is known as propagation delay and, as we said, becomes worse with the increase in the operating frequency (clock).

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Figure 2: Bits out of order arrive at the receptor due to the propagation delay problem.

The project of a bus using serial communication is much more simple to be implemented than one using parallel communication, since only two wires are necessary to data transmission (one wire for data transmission and one ground wire). Besides, serial communication allows operation with much higher clocks than those used in parallel communication, since problems with de electromagnetic interference and propagation delay appear most frequently in parallel communication, which prevents high clocks from being reached in the transmissions. Another difference between parallel communication and serial communication is that parallel communication is usually half-duplex (the same wires are used both to transmit and to receive data) due to the high number of wires that are necessary to its implementation, while serial communication is full-duplex (there’s a separate set of wires to transmit data and another one to receive data) because it needs just two wires.

That’s why engineers adopted serial communication instead of parallel communication in PCI Express bus.

Now you might be asking yourself: isn’t serial communication slower? Not necessarily, and the PCI Express bus is a good example: if higher clock is used, serial communication is faster than parallel communication.

We’ll talk about how the PCI Express bus works on the next page.