<p> PCI, PCI-X and PCI-E, What is the Difference? By Jon Busshart</p><p>I am sure that we are all familiar with the common PCI slot that has been with us for many years. Over the years the PCI slot has evolved into a PCI-X slot, which is mainly used in server and high performance workstations. You will see some key physical differences in the evolution of the slot. When we are talking to our customers, PCI-X and PCI Express roll off the tongue and often sound similar, but that is not the case as far as the physical slot is concerned. </p><p>In Figure 1, the short white slots should look pretty familiar. They are the common PCI slots. These slots are 32 bit and run at 33MHz. One thing you may have not noticed about these slots is that they have a 5 volt notch. I have one of the notches circled in green in Figure 1. We use these slots even today, but our servers and high performance workstations needed even more speed, so the 64 bit PCI slot was introduced. Circled in red, in Figure 1, is the 64 bit tail. So, the longer slot is a 64 bit slot and Figure 1 has a 5 volt notch. As time went by we needed to have even more speed, but we also needed to use less power. The industry boosted the slot to 66MHz and introduced the slot to only need 3 volts of power. The slots in Figure 2 should look pretty close to the slots in Figure 1, except for what I have circled in red. This is the 3 volt notch. The longer slot in Figure 2 is how our PCI-X slots physically look. Oh, by the way, with the introduction of the PCI- X slot, they jumped the frequency the slot runs at to 100MHz and 133MHz. PCI-X slots are backwards compatible, as all 64bit slots are, to our every day 32 bit PCI cards, as long as these shorter cards have one thing, the 3 volt notch. If we look at the PCI card in Figure 3 you will see it has both a 5 volt notch (green arrow) and a 3 volt notch (red arrow). This tells me that this card will physically fit into a PCI-X slot. Figure 2 PCI-X slots can push data at 1 Gigabits per second. As time marched on we needed to move our data faster. Also we needed a replacement for our aging AGP video slot, since AGP 8X was pushing the limits of the technology. In walks PCI Express (PCI-E or even PCIe). PCI Express is the serialization of the data bus. PCI Express slots are divided into lanes. As your PCI Express slot gets more lanes, the slots get longer. In Figure 4 you will see, in white, the common 32 Figure 3 bit PCI slot. Next to it is a 16x (16 lane) PCI Express Slot. The short guy right next to that is a 1x PCI Express slot. These slots come in favors of 1x, 2x, 4x, 8x and finally 16x slot (which we typically see new video card go into). This 1x single lane slot can push data at 250 Megabits per second. Taking advantage of full duplex, we can get 500 Figure 4 Megabits per second. This means a 16x slot can run at 8 Gigabits per second in full duplex mode. These new slots are not limited to high performance servers. As a mater of fact, they first started popping up in everyday desktops and workstations. They were then quickly adopted in our high performance workstations and servers. PCI Express slots are commonly found along side of PCI-X slots in these high end machines. The slot is flexible as well. You can install a smaller lane card into a larger lane slot. For example, you can install a 1x card into a 16x slot, or even a 4x card into an 8x slot. Even though PCI Express has PCI in its name, the slots are physically completely different from each other. There is no way to install a PCI Express card into a PCI or a PCI-X slot, and of course you can not install a PCI card into a PCI Express slot, as well.</p><p>As Processors, RAM, Hard Drives, Video Cards, as well as all I/O Cards increase in speed, so has the PCI slots grown in throughput. As I am typing this, the specification for PCI Express 2.0 is nearing completion, this will double the theoretical bandwidth a PCI Express lane can have. As with everything in our world, technology marches on to meet the ever increasing needs of the solutions that we provide to our customers. </p>