On-Board Ports, DMA Controller, Power Supply Connections, Purchasing A Systemboard

On-Board Ports

Many systemboards contain on-board ports such as a keyboard port and a mouse port.

In addition, a parallel printer port and one or two serial ports might be located directly on the systemboard.

Few older systemboards contain more ports than these. Some systems also have a video or network port, and newer systemboards contain one or two USB ports.

You don’t have to replace an entire systemboard if one port fails. Most systemboards contain jumpers or DIP switches that can tell the CPU to disable one port and look to an expansion card for the port instead. Ports can also be disabled through CMOS setup.

When buying a new computer or systemboard, look for the ability to disable ports, floppy drive connectors, or hard drive connectors coming directly from the systemboard by changing the hardware configuration.

You can easily tell if ports on the outside of the case are directly connected to the systemboard without opening the case; the ports are lined up along the bottom of the computer case, as shown in the figure below.

DMA Controller

The DMA (Direct Memory Access) controller on a systemboard is part of the chip set and provides faster memory access because it handles the movement of data in and out of RAM without involving the CPU. The DMA chip is also responsible for dynamic memory refreshing.

Power Supply Connections

The systemboard contains connections to receive power. Two connections are found on the edge of the systemboard for the power supply. Voltages are sometimes written on the systemboard for each pin. Voltages for most systemboards are +5v, –5v, +12v, and –12v.

Purchasing A Systemboard

When you buy a systemboard, your selection should be based on the following components:

Types and speeds of the CPU you can use Chip set on the board (already installed) Memory cache type and size
Types and number of expansion slots: EISA, PCI, and AGP
Type of memory: ECC, EDO, SDRAM, SIMMs, or DIMMs
Maximum amount of memory you can install on the board and the incremental amounts by which you can upgrade memory
Type of case you can use ROM BIOS (already installed)
Type of keyboard connector
Presence or absence of different types of proprietary video and/or proprietary local bus slots
Presence or absence of IDE adapters and SCSI controller
Presence or absence of COM ports, LPT ports, and mouse port

Sometimes a systemboard contains a component that is more commonly offered as a separate device. A component on the board is called an embedded component. One example is support for video. The video port might be on the systemboard or might require a video card. The cost of a systemboard with an embedded component is usually less than the combined cost of a systemboard with an expansion card but no component. If you plan to expand, be cautious about choosing a proprietary board that has many embedded components. A proprietary design using many embedded devices often does not easily accept add-on devices from other manufacturers. For example, if you plan to add a more powerful video card, you might not want to choose a systemboard that contains an embedded video controller.