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The Personal Computer Motherboard

The motherboard was a new idea when microcomputers and the PC first started gaining popularity. Before integrated circuits, individual components of the computer were housed on different boards or different units housing many boards. Nowadays, the components a computer needs to function are on the motherboard. Many motherboard manufacturers also take this a step further and have integrated some more common expansion cards on to the motherboard, such as graphics devices, sound cards, networks card, disk controllers etc.

The motherboard forms part of the support for the main components, the processor, memory and I/O (input / output) and is housed inside the case. Two major factors define a motherboard, 1) the processor type and speed, 2) the bus architecture. Motherboards have changed with each introduction of new processors and bus architectures. Faster processors offer better performance and newer bus architectures have increased the data transfer rate. The original PC bus was 8 bits wide and could transfer 1 MB per second, the 16 bit ISA bus transfers 8 MB / s, VESA bus (32 bit) offers 150 MB /s, PCI 32 or 64 bits supports 264 MB / s and the newer AGP 64 bit bus supports over 528 MB per second transfer rates. Extending of the bus architecture was done in a completely backward compatible manner. Adaptors designed for the original IBM PC should still work in a modern Pentium system. The expansion bus architecture is an extension of the processors local bus and chips are added to support direct memory access (DMA), Interrupts, counters, timer, caching and bridges. DMA circuitry is used to transfer data at high speed between an expansion card and memory. Interrupts enable the computer to interrupt what it is doing and do something else. All ISA bus systems come with the Intel chips that were designed for the original IBM PC. The processor (CPU) on a modern designed motherboard is probably the most important component and fit in a socket rather than soldered to the board. The socket is designed to accept several processor types and speeds and allows the system to be upgraded with a more powerful CPU. The socket is designed so that minimal pressure if any is required to insert the processor and the chip is secured with a locking lever that grips the processor pins in the socket. There are motherboards that support more than one processor on the same board.

The processor is attached to the system memory (DRAM Dynamic Random Access Memory) by a bridge. Current memory is unable to keep up with modern high-end processor and the bridge meets timing, interface and refreshing requirements of the DRAM devices. A second, smaller memory called cache is placed between the processor and DRAM. Cache memory is far faster and modern Pentium processors have cache on the same die and can keep up with the processors speed. Traditionally cache was designed using static RAM (SRAM), installed in small blocks and managed by a cache controller. The DRAM controller on modern boards is usually on the same chip as the cache controller. The cache controller loads the SRAM from DRAM thus making data and instructions available to the processor at the highest speed. The processors local bus is connected to the PCI and ISA buses with bridge chips. The PCI bus bridge chip changed design with each processor change. The ISA bridge contains all the core design for the original IBM PC AT 16 bit bus, including 8237 DMA controller, 8259 Interrupt controller and 8254 timer/counter chips. This chip allows backward compatibility with older expansion devices and interfaces.

Since the original PC a keyboard controller device and port is incorporated on the motherboard. Newer designs support serial ports integrated on the motherboard. The keyboard uses a serial interface adaptor to transmit and receive data 1 bit at a time until a full block or command is transmitted. The keyboard and controller are isochronous, meaning the data and clock are synchronized, on the serial clock line, on the edges of a clock signal. (The adaptor device sets a bit on a serial data line then toggles a serial clock line on the adaptor, each toggle of the serial port line the keyboard responds by sending data to the motherboard). Traditionally keyboards attached to a 5-pin DIN connector at the rear of the system unit, and recently termed AT form. PS/2 keyboard ports connect using a 6-pin mini din adaptor. PS/2 ports are found on all modern ATX form motherboards. The 8042 chip supports a second auxiliary device, typically a PS/2 mouse. The keyboard serial clock and data interface signals attach to an Intel 8042 chip that in turn connects to an 8 bit bus the sub-ISA or X bus that connects to the local bus by using a PCI to ISA bridge chip (mentioned earlier). The 8042 micro-controller sends and receives commands, status and data to the processor using interrupts level 1 for the keyboard and level 12 for the PS/2 mouse if supported. The keyboard also has a controller that attaches to the clock and data signals to communicate with the keyboard controller.

The ROM BIOS (Read Only Memory Basic Input Output System) is a device that contains software that primarily loads the basic operating system software from the boot-up device and performs power on self-tests (POST). Newer BIOS support Plug and Play (PnP), ACPI (Advanced Configuration Peripheral Interface), USB (Universal Serial BUS), software power-off and AGP (Accelerated Graphics Port), an new expansion port on the Host to PCI bridge device. The main purpose of the AGP port is to provide fast access to system memory. The AGP port supports 2X and 4X data rate transmitting at 66.6*2*4 (533 Mb/Sec) and 66.6*4*4 (1.06 Gb/Sec), a read queue used to pipeline reads, reducing read latency and side band addressing to transmit the data address on a separate line in order to further speed the transfer.

Every motherboard since the original IBM PC (apart from some earlier Amstrad clones) has a Real Time Clock (RTC) and Calendar chip a device that also contains a small amount of non-volatile memory. Even when the system is powered off this device is always running, by battery power, also installed on the motherboard. The RTC/Calendar is relatively accurate and provides date and time data to the operating system and. The device also stores basic configuration data in non-volatile memory. Disconnecting the battery or a low battery condition will loose the configuration information.

Many motherboards now have on board support for Enhanced (Intelligent Drive Electronics) ATA-2 ports. There have been several enhancements of this interface that allow for support of four drives and a massive increase in data transfer. The hard drive capacity limitation has been increased and CD ROM drive support through a standard called ATAPI (AT Attached Packet Interface) was added. Several data transfer modes and speeds have been designed, standard PIO (Programmed I/O) and single word and multiword DMA transfer. The single word DMA transfers 16 bits of data per DMA transfer request and multi word DMA transfers 16 bits continuously while the DMA signal is active. On modern motherboards all 4 IDE channels are capable of bus mastering and the boot-up can be set to options other than A: C: or C: ,A:. Along with IDE support modern motherboards typically contain two serial communication ports supporting standard asynchronous transmission protocols over RS 232 C interface ports. Over the years PC serial ports have upgraded with a series of UART’s (Universal Asynchronous Receiver Transmitter) devices, the latest NS16550 provides 16 byte FIFO buffers on transmit and receive data and limits the PCs need to service every data transaction by servicing (interrupts) in blocks. The device supports a maximum data rate of 115 Kb /sec transfer. Nearly every modern PC also comes with an on board parallel port and adaptor. The port is known by several names including, centronics, parallel, LPT, PRN, IEEE 1284 port and parallel printer port. The original PC supported an 8 bit printer port with maximum transfer rate of 150 KB / sec. The interface was enhanced and permitted 8 bit directional data transfer and enabled non printer devices to be attached. The parallel port was further enhanced with additional high speed transfer modes and further enhancing the interface protocol and supporting command cycles, data cycles and multiple logical device addressing.

Another device featured on motherboards is infrared port and will be the subject of another page. For the moment most motherboards are not bought with Infrared devices, some laptops do however feature it. The IrDA port utilises the resources of one COM port if enabled. (See Integrated Peripherals to configure the IrDA port field in the ROM BIOS.

Level two caches were required when processor speeds reached 50 and 66 MHz. The small cache on the processor was called level one and the external large cache level two. Level one cache is filled from level two cache which is filled from main memory (DRAM). Level two caches are typically 256 KB. Increasing cache size has diminishing effects over 1 MB.

Intel encouraged systems designers to place sockets on the motherboards, so that a PC could be upgraded by unplugging the older processor and replacing it. Some systems had two sockets so that the old processor did not even need removed. Intel gave away processor sockets to PC manufacturers and the clone processor market really took off and some manufacturers offered faster and better upgrades. The main point is the PC would never be obsolete.

The ATX Power Connector on the motherboard provides socket connection to a single 20 pin plug from the ATX PSU (Power Supply Unit) and supports the ACPI specification The are several new functions and connections that work with the ATX power supply. Software Power-Off, Modem Ring Power-On and Alarm Wake Up. An ATX case power button connects to the motherboard and not the PSU (like traditional AT cases) serving as a soft switch such that holding the button for more than 4 seconds will turn the system completely off, otherwise the button acts as a suspend mode switch. The software power-off control functions in windows 9x. Clicking 'Start' on the task bar then selecting 'Shut Down The Computer' does not display the message 'It is now safe to turn off your computer' or then wait for you to do so, the computer will be powered off after Windows completely exits. While in soft-off or suspend mode and a external modem ring signal occurs, the system wakes up. Alarm wake-up auto boots the system at a certain time. All the options must be enabled in the BIOS setup Power Management section.

Many modern motherboards support onboard hardware monitoring circuits that monitor the system temperatures, voltages and fan speeds. With the use of system management utilities, the hardware status can be checked and warnings given when there is a problem. In order to enable the CPU overheat protection feature, the Power Management field in the ROM BIOS should be enabled.

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Last modified: December 20, 2000