Buses: ISA, AGP, PCI, PCI Express, AMR


Buses (or, in English, bus) are, in a few words,patterns of communication used in computers for the interconnection of the various devices. In this article,you will get to know some characteristics of the main buses present in the PCs, such as ISA, AGP, PCI, PCI Express , and AMR. Note that many of these standards no longer used on new computers, even so, get to know them is important.

Before we begin, it is important for you to know that,in the course of this text, the AbbreviationFinder will use with some frequency the word slot. This term makes reference to the fittings physical of each bus for the connection of devices (video cards,network adapters, etc.). In general, each bus has a type of slot different.

ISA bus (Industry Standard Architecture)

The ISA bus is a standard no longer used, being found only in older computers. His appearance gave at the time of the IBM PC and this first version works with transfer 8-bit and clocked at 8.33 MHz (in fact, before the emergence IBM PC-XT, this value was 4,77 MHz).

At the time of the emergence of processor 286, the ISA bus won a version capable of working with 16-bit. Previous devices working with 8-bit functioned normally in the slots with the default 16-bit, but otherwise it was not possible, this it is, of ISA devices 16-bit to work with slots of 8-bit, because the slots 16-bit ISA had an extension that make them larger than 8 bits, as shown in the image below:

Notice in the image above that the slot contains a division. The cards 8-bit use only the largest part. As you have already should have guessed, the cards is 16-bit use both of the parties. By accountalso, the motherboards of that time started to count with only slots 16-bit ISA. Interestingly, some models have been released having both slots of 8-bit and slots of 16 bits.

If you are familiar with slots the latest, surely realize how big are the fittings ISA. The 16-bit, for example, account for 98 terminal. So, it is possible to notice that the expansion cards of the season (that is, video cards, sound cards, modem cards, etc.) were also great. Despite this, it was not difficult to find cards that did not use all of the contacts of the ISA slots, leaving a plenty of space in the dock.

With the evolution of technology, the standard ISA has been gradually losing space. The 16-bit version is capable of providing data transfer in the house of 8 MB per second, but it is unlikely that this value is reached, getting around 5 MB. As this rate of transfer was sufficient for certain devices (modem cards, for example), for some time it was possible to find motherboards that had both with ISA slots and PCI slots (the default successor).

PCI bus (Peripheral Component Interconnect)

The PCI bus has emerged in the early 1990s by the hands of the Intel. Its main features are the ability transfer data at 32-bit and clocked at 33 MHz, specifications these that have become the standard capable of transmitting data at a rate up to 132 MB per second. The PCI slots are smaller than the ISA slots, as well as their devices, obviously.

But, there is another feature that has become the standard PCI attractive: the feature Bus Mastering. In a few words, it is a system that lets devices that make use of the bus read and write data directly in RAM, without the processor has that “stop” and interfere to make this possible. Note this feature is not unique to the PCI bus.

Another striking feature of PCI is its compatibility with the feature of Plug and Play (PnP), something like “plug and use”. With this functionality, the computer is able to recognize automatically the devices that are connected to the PCI slot. Currently, such ability is trivial in the computers, that is, just connect the device, turn the computer on and wait for the operating system to warn on the recognition of a new item so that you can install the proper drivers (this if the operating system does not install it alone). In the old days, the computers did not work that way and the emergence of the Plug and Play feature was a revolution in this sense. In addition to being used in buses today, this functionality came to be implemented in standards and older, even in ISA.

The PCI bus also underwent developments: a version that works with 64-bit and 66 MHz has been released, taking also an extension in its slot. Their maximum rate data transfer is estimated at 512 MB per second. Despite this, the standard PCI 64-bit never come to be popular. One of the reasons for this is the fact that specification generate more costs to the manufacturers. In addition, the majority of the devices from the time of the peak of the PCI not needed data transfer rates higher.

Bus PCI-X (Peripheral Component Interconnect Extended)

A lot of people confuse PCI-X with PCI Express standard (shown below), but both are different. The PCI-X nothing more than an evolution of the PCI 64-bit, being compatible with the earlier specifications. Version PCI-X 1.0 is capable of operating at frequencies of 100 MHz and 133 MHz. In this last, the standard can attain the rate data transfer 1.064 MB per second. The PCI-X 2.0, by turn, can also work with the frequencies of 266 MHz and 533 MHz.

AGP bus (Accelerated Graphics Port)

Before computers were limited to displaying only the characters in the screens dark, today they are able to view and create images in very high quality. But, this has a price: the more evolved you are a graphical application, in general, the more data it will consume. To cope with the increasing volume of data generated by the graphics processors, Intel announced in mid-1996 the standard AGP, whose slot serves solely to video cards.

The first version of the GPA call (AGP 1.0) works to 32-bit and is clocked at 66 MHz, which is equivalent to a transfer rate data up to 266 MB per second, but in truth, you can get the value of 532 MB per second. Explains: the AGP 1.0 can work in 1x mode or 2x. With 1x, a given pulse of the clock is transferred. With 2x, there are two data pulse clock.

In mid-1998, Intel released the AGP 2.0, whose differential are in the possibility to work also with the new mode operation 4x (offering a transfer rate of 1.066 MB per second) and power supply 1.5 V (AGP 1.0 works with 3.3 V). Some time after came up the GPA 3.0, with the ability to work with power electric 0.8 V and mode of operation of 8x, corresponding the a transfer rate of 2.133 MB per second.

In addition to the high data transfer rate, the default AGP also offers other advantages. One of them is the fact always able to operate at its maximum capacity, as there is no other device on the bus that can, in some way, interfere with communication between the video card and the processor (remember that the AGP is only compatible with video cards). The GPA also allows the board of video make use of part of the RAM memory of your computer as a increase of its own memory, a feature called Direct Memory Execute.

As for the slot, the AGP is slightly smaller than a PCI docking. However, as there are several versions of the GPA, there is variations in the slots (which is unfortunate, because this creates a lot of confusion). These differences occur mainly because of the power settings electric existing between the devices that use each version. There is, for example, a slot that works for AGP 1.0, the other that works for AGP 2.0, a third party who works with all versions (slot universal), and so on. The illustration below shows all the types of connectors:

As you may have noticed in the image above, the market also met special versions of the GPA calls AGP Pro, directed the video cards that consume a large amount of energy.

Despite some advantages, the standard GPA ended up losing space and has been replaced by the PCI Express bus.

PCI Express bus

The standard PCI Express (or PCIe or even PCI-EX) it was designed by Intel in 2004 and stands by to replace, at the same time, the PCI and AGP buses. This happens because the PCI Express is available in various segments: 1x, 2x, 4x, 8x and 16x (there are also the 32x, but up to the closing of this article, this was not in use by industry). The higher this number, the greater is the rate of data transfer. As shown in the image below, this division is also reflected in the size of the slots PCI Express:

The PCI Express 16x, for example, is able to work with rate transfer about 4 GB per second, characteristic that makes be used for video cards, one of the devices that generate most data on a computer. The PCI Express 1x, the same being the more “weak”, is able to achieve a rate of download of about 250 MB per second, a value sufficient for a good part of devices more simple.

With the release of PCI Express 2.0, which happened in the beginning 2007, the rates of transfer of technology almost doubled.

Learn more about the PCI Express technology in this an article published here in the AbbreviationFinder.

Buses AMR, CNR and ACR

Standards AMR (Audio Modem Riser), CNR (Communications and Network Riser) and ACR (Advanced Communications Riser) are different from each other, but share the idea of allowing the connection to the motherboard of devices to the Host Signal Processing (HSP), that is, devices whose control is done by the processor of the computer. For this, the chipset on the motherboard needs to be compatible. In general, these slots are used for cards that require little processing, such as sound cards, network cards or modem cards, simple.

The slot AMR has been developed to be used especially for functions modem and audio. Your project was spearheaded by Intel. To be used, the chipset of the motherboard I had to tell the circuits AC’97 and MC’97 audio and modem, respectively). If compared to the standards seen up to now, the slot AMR is very small:

The default CNR, in turn, arose virtually as a substitute of the AMR and also has the Intel as the main name in your development. Both are, in fact, very much alike, including in the slots. The the main difference of the CNR is to support the network resources, in addition to of the audio and modem.

In relation to the ACR, it is a standard whose development main name AMD. Its main focus are the communications network and USB. This type was for some time common to be found in motherboards from ASUS and your slot is extremely similar to a plug-in PCI, with the difference of being positioned in the opposite way on the motherboard, or is, is a sort of “PCI inverted”.

Other buses

The buses mentioned in this text have been or are widely used by the industry, but there are many patterns that for various reasons, were acceptance of the more limited in the market. This is the case, for example, of the buses VESA, MCA, and EISA:

VESA: also called VLB (VESA Local Bus), this the standard was established by the Video Electronics Standards Association (hence the acronym VESA) and it works physically, as an extension of the standard ISA (there is a fitting after an ISA slot on motherboards that are compatible with the default). The VLB can work the 32-bit and with the frequency the external bus of the processor (at the time, the default was of 33 MHz), causing your data transfer rate he could reach up to 132 MB per second. Despite this, the technology did not last long, especially with the arrival PCI bus;

MCA: abbreviation for Micro Channel Architecture, MCA was conceived by IBM to be the replacement for the standard ISA. This technology works at the rate of 32 bits and a frequency of 10 MHz, in addition to be compatible as features such as Plug and Play and Bus Mastering. One of the obstacles that contributed to the non-popularization the MCA was the fact of this being a bus owner, this is, belonging to the IBM. On this account, interested parties in technology they had to pay royalties to place it in their products, the idea that, obviously, was not well received;

EISA: acronym for Extended Industry Standard Architecture, the EISA is, as the name indicates, a bus compatible with the technology ISA. Because of this, you can operate the 32-bit, but keeps their frequency in 8.33 MHz (the same as in the ISA). Your slot is virtually identical to that of the standard ISA, however, is more high, since it uses two rows of contacts: the first is destined for the ISA devices, while the second serves to the devices 32-bit.

Buses Isa, Agp, Pci, Pci Express, Amr and Other 1

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