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PC bus systems

Any data transfer network is only as strong as the weakest link. We have seen an increase in PC performance in terms of processor speed, memory access speed, bus speed and architectures, and although these issues are becoming less critical in many application areas, they still need to be considered when a PC's CPU loading is high.

Based on our substantial experience, we have a detailed understanding of the various factors that affect a PC's performance in demanding machine vision applications. We are able to supply preconfigured, fully tested PCs, with the acquisition interface included, ensuring that our customers always receive a system that exactly meets their performance requirements. All systems provided by STEMMER IMAGING are validated to work at demanding data rates with all our acquisition interface boards. Historical evidence concludes that many PCs suitable for general computing are just not designed or tested to work at the data rates demanded by machine vision. Our service reduces the time needed for the integration of a system. You can be secure in the knowledge that we manage the life cycle of our systems, ensuring we can deliver compatible solutions when the PC reaches its end of life. 8.4.1 PC bus architectures Since the development of the first PC/AT interface system architectures have developed significantly. Firstly the PCI bus and now the PCI Express bus. As the ISA bus is no longer used, it will not be discussed here. The section should help you to select the correct interface that will deliver the required performance. Currently we see PCs with both PCI and PCI Express interfaces, however, it is obvious that PCI Express is becoming the dominant interface while the importance of the PCI bus diminishes in new designs.

PCI bus

Many people imagine that the PCI bus is universally compatible; however, older PCI cards used 5 V signalling whereas new motherboards only support 3.3 V signalling. As the following graphic shows, all devices on a PCI bus use the same data channel whereby the available bandwidth has to be divided among them.

PCI Express

PCI Express (Peripheral Component Interconnect Express) is an expansion standard for connecting peripheral devices to the chipset of the main processor. PCI Express is the successor of PCI and offers a higher data rate per pin. PCI Express or PCIe as it is sometimes called, serialises the data into 'lanes'. The terminology used for each lane is x1 for 1 lane while x4 is 4 lanes. Currently x1, x4, x8 and x16 can be found on standard PCs with the possibility of x32, x48 and x64 reserved for use on servers. As the data is serialised, only 4 pins are required for each lane, which doubles the data rate with only a small increase in the connector footprint. In addition, the bus can support maximum data transfers in both directions simultaneously.

In a system with multiple PCIe slots the bus works in a point-to-point way, delivering the full bandwidth to each device, unlike standard PCI which shares the data rate across all slots. On the PCI Express bus, each device has a dedicated 'lane' that transfers the data to and from a switch, which behaves in a similar way to a switch on a network. When the PC starts up, PCI express identifies all attached devices and builds a map that is then used to arbitrate between all the different data streams.

Compatibility is such that a card with fewer lanes than the slot it is plugged into, will work albeit at the maximum rate of the card. However, it is not possible to fit a card with more lanes into a slot with fewer lanes.

Other advanced features of the PCIe bus include advanced power management, support for real-time data traffic, hot plug/hot swap, data integrity and error handling. As the development of electronic signalling increases clocking speeds, we are now seeing new versions of the PCIe increasing the throughput per lane.

The following table shows the theoretical data rates on PCI Express:

PCIe 1.0 PCIe 2.0 PCIe 3.0 PCIe 4.0
x1 250 MB/s 500 MB/s 1,000 MB/s 2,000 MB/s
x2 500 MB/s 1,000 MB/s 2,000 MB/s 4,000 MB/s
x4 1,000 MB/s 2,000 MB/s 4,000 MB/s 8,000 MB/s
x8 2,000 MB/s 4,000 MB/s 8,000 MB/s 16,000 MB/s
x16 4,000 MB/s 8,000 MB/s 16,000 MB/s 32,000 MB/s
x32 8,000 MB/s 16,000 MB/s 32,000 MB/s 64,000 MB/s

Thunderbolt

Thunderbolt, developed under the name Light Peak is a hardware interface that allows the connection of external peripherals to a computer.

Thunderbolt combines PCI Express (PCIe) and DisplayPort (DP) into one serial signal alongside a DC connection for electric power, transmitted over one cable, using the same connector as Mini DisplayPort (MDP). Up to six peripherals may be supported by one connector through various topologies.

Over time this is expected to become a standard interface for computer components.