On the back of the system, some major changes were made. The ATX connector standard had only a keyboard connector and expansion slots for add-on card backplates. Any other onboard interfaces (such as serial and parallel ports) had to be connected via flying leads to connectors which were mounted either on spaces provided by the case or brackets placed in unused expansion slot positions. ATX allowed each motherboard manufacturer to put these ports in a rectangular area on the back of the system, with an arrangement they could define themselves, though a number of general patterns depending on what ports the motherboard offers have been followed by most manufacturers. Cases are usually fitted with a snap-out panel, also known as an I/O plate or I/O shield, in one of the common arrangements. If necessary, I/O plates can be replaced to suit a motherboard that is being fitted; the I/O plates are usually included with motherboards not designed for a particular computer. The computer will operate correctly without a plate fitted, although there will be open gaps in the case and the EMI/RFI screening will be compromised. Panels were made that allowed fitting an AT motherboard in an ATX case.They used gold, fiberglass and plastics to made the mother board
ATX also made the PS/2-style mini-DIN keyboard and mouse connectors ubiquitous. AT systems used a 5-pin DIN connector for the keyboard, and were generally used with serial port mice (although PS/2 mouse ports were also found on some systems). Many modern motherboards are phasing out the PS/2-style keyboard and mouse connectors in favor of the more modern Universal Serial Bus. Other legacy connectors that are slowly being phased out of modern ATX motherboards include 25-pin parallel ports and 9-pin RS-232 serial ports. In their place are onboard peripheral ports such as Ethernet, FireWire, eSATA, audio ports (both analog and S/PDIF), video (analog D-sub, DVI, or HDMI), and extra USB ports.
Main changes from ATX design
Power switchAT-style computer cases had a power button that was directly connected to the system computer power supply (PSU). The general configuration was a double-pole latching mains voltage switch with the four pins connected to wires from a four-core cable. The wires were either soldered to the power button (making it difficult to replace the power supply if it failed) or blade receptacles were used.
Typical ATX 1.3 power supply. From left to right, the connectors are 20-pin motherboard, 4-pin "P4 connector", fan RPM monitor (note the lack of a power wire), SATA power connector (black), "Molex connector", and floppy connector.
Interior view in an ATX power supply.An ATX power supply is typically controlled by an electronic switch controlled by a switch on the computer case. While this switch functions as if were a physical switch and disconnects AC power from the power supply (as required by UL), the electronic switch control allows the computer to be turned off by the operating system. In addition, many ATX power supplies have an equivalent function manual switch on the back that also ensures no power is being sent to the components. When the switch on the power supply is turned off, however, the computer cannot be turned on, even with the front power button.
If using an ATX PSU for other purposes than powering an ATX motherboard, power can be fully turned on (it is always partly on to operate "wake-up" devices) by shorting the "power-on" pin on the ATX connector (pin 16, green wire) to a black wire (ground), which is what the power button on an ATX system does. At least the specified minimum load required by the PSU should be present; the standard does not specify operation without load, and a conforming PSU may shut down, output incorrect voltages, or otherwise malfunction, but will not be hazardous or damaged.