standard supports bus bridging: as many as ten daisy chained PCI buses have been tested. Cardbus, using the PCMCIA connector, is a PCI format that attaches peripherals to the Host PCI Bus via PCI to PCI Bridge. Cardbus is being supplanted by ExpressCard format. Intel introduced the AGP bus in 1997 as a dedicated video acceleration solution. AGP devices are logically attached to the PCI bus over a PCI-to-PCI bridge. Though termed a bus, AGP usually supports only a single card at a time (Legacy BIOS support issues). From 2005 PCI Express has been replacing both PCI and AGP. This standard, approved in 2004, implements the logical PCI protocol over a serial communication interface. PC/104(-Plus) or Mini PCI are often added for expansion on small form factor boards such as Mini-ITX. For their 1000 EX and 1000 HX models, Tandy Computer designed the PLUS expansion interface, an adaptation of the XT-bus supporting cards of a smaller form factor. Because it is electrically compatible with the XT bus (a.k.a. 8-bit ISA or XT-ISA), a passive adapter can be made to connect XT cards to a PLUS expansion connector. Another feature of PLUS cards is that they are stackable. Another bus that offered stackable expansion modules was the "sidecar" bus used by the IBM PCjr. This may have been electrically comparable to the XT bus; it most certainly had some similarities since both essentially exposed the 8088 CPU's address and data buses, with some buffering and latching, the addition of interrupts and DMA provided by Intel add-on chips, and a few system fault detection lines (Power Good, Memory Check, I/O Channel Check). Again, PCjr sidecars are not technically expansion cards, but expansion modules, with the only difference being that the sidecar is an expansion card enclosed in a plastic box (with holes exposing the connectors). External expansion buses Laptops are generally unable to accept most expansion cards intended for desktop computers. Consequently, several compact expansion standards were developed. The original PC Card expansion card standard is essentially a compact version of the ISA bus. The CardBus expansion card standard is an evolution of the PC card standard to make it into a compact version of the PCI bus. The original ExpressCard standard acts like it is either a USB 2.0 peripheral or a PCI Express 1.x x1 device. ExpressCard 2.0 adds SuperSpeed USB as another type of interface the card can use.