Backplane
The term backplane , bus board , backplate or back wiring unit is used in various forms in electronics. On the one hand, it is a technical structure in electronic devices to add further electronic assemblies. On the other hand, a special part of the infrastructure of computer networks is so called.
Use in electronic devices
Backplane is the name given to the rear wall of an electronic device that has several slots for plug-in modules ( plug-in cards ). The backplane carries the connectors for the plug-in modules and connects them electrically. As a rule, it is a passive component, so there are no active components such as ICs or passive components such as resistors or capacitors on it.
In the early days of electronics, backplanes consisted of racks in which connectors for plug-in cards were mounted next to one another. The electrical connections of the individual pins between the connectors were still made by hand and freely in the air, mostly in order in bundled cable strands (see figure below), but also completely free in other systems (example: early Cray models). In addition, winding connections were also used.
Hand-wired backplane from the 1950s
Backplane wiring of a Cray-2
Backplanes as mass-produced goods are designed as multi-layer circuit boards . In order to keep contact resistances low, bores are used during production into which the connectors are simply pressed . Since it is still not populated, it does not run through the solder bath . In the manufacture of devices, it is therefore a finished part that is supplied by companies that specialize in the manufacture of printed circuit boards, not the assembly of printed circuit boards.
The electrical connections that the backplane establishes between the slots can be made very differently and determine the architecture of the device. If the connections lead from several slots of the same type to a central assembly, we speak of star-shaped wiring . The individual assemblies cannot then communicate with one another, but only via the central assembly. If assemblies of the same type are connected to one another, this represents a bus . This type of connection via a backplane requires a bus arbitration process , a control that prevents different assemblies from writing to the bus at the same time. This process determines the speed of data traffic on the bus and is usually a device identifier. If, for example, the performance of a router is described with a throughput of 1 GBit / s, this usually means that the bus on the backplane can produce this data throughput.
The VMEbus is an older bus system for backplanes .
Backplanes are often also used in the computer area in servers and mainframes. One (or more) plug-in card (s) with processors is plugged onto the backplane. You can then z. B. Use PCI , ISA or other interfaces.
Backplane-like circuit boards for bus systems that are plugged onto a main board are riser cards . LPX and NLX were form factors that led bus systems for plug-in cards from the motherboard.
Use in switches
A backplane is also a high-speed bus for switches . A switch controls communication in computer networks. The individual ports of a switch are connected via the backplane, whereby this connection differs from the matrix switches . In contrast to the matrix architecture, not all ports are connected to each other, but are located on a data bus.
A backplane can have a certain data transfer rate . B. in gigabits per second (Gbps or Gbit / s). The value of the data transmission rate is important in the case of expected heavy data traffic in a network. If all ports of a switch are used in full-duplex mode, e.g. B. 8 ports with 100 Mbit / s times 2, then the data rate of the backplane should be at least 1.6 Gbps. Otherwise there is a blocking (English blocking ) of individual data packets. A switch architecture that is designated as non-blocking therefore has an available data transfer rate that is higher than the maximum possible data traffic. The full data transmission rate can be made available to each port at any time.