A portable computer is a computer that is designed to be moved from one place to another (in other words, it is a computer that is portable). Portable computers, by their nature, are microcomputers. Early portables were unkindly referred to as "luggables," referring to their great size and weight (owing partly to the need to include a full-blown CRT monitor, as LCD technology was not yet mature). The term "luggable" is today used mainly when speaking of 17" and larger widescreen laptops.
Battery Solutions
Nickel Cadmium – NiCd batteries were actually the first rechargeable laptop batteries ever. Manufactures loved them because their cost was relatively low and they had a high output. You won’t find Nickel Cadmium batteries being used anymore, due to them being heavier and not as efficient as the newer laptop batteries.
Nickel Metal Hydride – NiMH batteries can still be found all over the place -- particularly for older model laptops. The rechargeable NiMH laptop battery was a big step up for notebook technology mostly in part because they were more reliable than the NiCd batteries, and they had an even higher output. The NiMH battery was also cheaper to produce, and safer to use.
The only issue with NiMH batteries is that they can have a memory effect. Basically, if you don’t fully discharge the battery, it can remember this and leave you with a less than perfect battery output.
Lithium Ion – LiON batteries are now used in most new laptops. Unlike the NiMH battery, LiON laptop batteries have no memory effect. LiON batteries are also lighter than both NiCd and NiMH notebook batteries. Both of these advantages equal out to the Lithium Ion battery being the most popular and most expensive among the various notebook power sources.
Lithium-ion is designed to operate safely within their normal operating voltage but become unstable if charged to higher voltages. When charging above 4.30V, the cell causes plating of metallic lithium on the anode; the cathode material becomes an oxidizing agent, loses stability and releases oxygen. Overcharging causes the cell to heat up. If left unattended, the cell could vent with flame.
Much attention is focused to avoid over-charging and over-discharging. Commercial lithium ion packs contain a protection circuits that limit the charge voltage to 4.30V/cell, 0.10 volts higher than the voltage threshold of the charger. Temperature sensing disconnects the charge if the cell temperature approaches 90°C (194°F), and a mechanical pressure switch on many cells permanently interrupt the current path if a safe pressure threshold is exceeded. Exceptions are made on some spinel (manganese) packs containing one or two small cells.
Extreme low voltage must also be prevented. The safety circuit is designed to
cut off the current path if the battery is inadvertently discharged below
2.50V/cell. At this voltage, most circuits render the battery unserviceable and
a recharge on a regular charger is not possible.
There are several safeguards to prevent excessive discharge. The equipment
protects the battery by cutting off when the cell reaches 2.7 to 3.0V/cell.
Battery manufacturers ship the batteries with a 40% charge to allow some
self-discharge during storage. Advanced batteries contain a wake-up feature in
which the protection circuit only starts to draw current after the battery has
been activated with a brief charge. This allows prolonged storage.
The first microcomputer to feature a slot-type expansion card bus was the Altair 8800, developed 1974-1975. Initially, implementations of this bus were proprietary (such as the Apple II and Macintosh), but by 1982 manufacturers of Intel 8080/Zilog Z80-based computers running CP/M had settled around the S-100 standard. IBM introduced the XT bus, with the first IBM PC in 1981; it was then called the PC bus, as the IBM XT, using the same bus (with slight exception,) was not to be introduced until 1983. XT (a.k.a. 8-bit ISA) was replaced with ISA (a.k.a. 16-bit ISA,) originally known as AT bus, in 1984. IBM's MCA bus, developed for the PS/2 in 1987, was a competitor to ISA, also their design, but fell out of favor due to the ISA's industry-wide acceptance and IBM's closed licensing of MCA. EISA, the 16-bit extended version of ISA championed by Compaq, was common on PC motherboards until 1997, when Microsoft declared it a "legacy" subsystem in the PC 97 industry white-paper. VESA Local Bus, an early 1990s expansion bus that was inherently tied to the 80486 CPU, became obsolete (along with the processor) when Intel launched the Pentium CPU in 1993.
The PCI bus was introduced in 1991 as replacement for ISA. The standard (now at version 3.0) is found on PC motherboards to this day. Intel introduced the AGP bus in 1997 as a dedicated video acceleration solution. Though termed a bus, AGP supports only a single card at a time. From 2005 PCI-Express has been replacing both PCI and AGP. This standard, approved [by who?] in 2004, implements the logical PCI protocol over a serial communication interface.
After the S-100 bus, this article above mentions only buses used on IBM-compatible/Windows-Intel PCs. Most other computer lines that were not IBM compatible, including those from Tandy, Commodore, Amiga, and Atari, offered their own expansion buses. Even many video game consoles, such as the Sega Genesis, included expansion buses; at least in the case of the Genesis, the expansion bus was proprietary, and in fact the cartridge slots of many cartridge based consoles (not including the Atari 2600) would qualify as expansion buses, as they exposed both read and write capabilities of the system's internal bus. However, the expansion modules attached to these interfaces, though functionally the same as expansion cards, are not technically expansion cards, due to their physical form/
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 the same as or similar 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.)