Motherboard Troubleshooting

The motherboard is the heart of any personal computer. It is the motherboard that handles system resources (IRQ lines, DMA channels, I/O locations) as well as "core" components such as the CPU, math coprocessor, and all system memory, including DRAM, BIOS ROM, cache, and CMOS RAM. Indeed, most of a PC's capabilities are defined by motherhboard components. This chapter shows you how to recognize symptoms and translate error information into motherboard repair. You will typically find the following elements on your motherboard:

• CPU (central processing unit). The CPU is a programmable logic device that performs all of the instruction, logic, and mathematical processing in the PC. This is the single most important IC in the computer CPU failure can disable the entire PC.
• MPC (math coprocessor). The MPC is a programmable logic device (closely related to CPU) which is tailored for handling floatingpoint math operations. Math-intensive application software which is written to take advantage for the MCP can realize substantial improvements in performance. All contemporary CPIs now integrate math coprocessor functions without the need for a seperate MCP.
• BIOS (basic input /output system). This is the code used to control the motherboard's hardware and perform low-level motherboard operations. Traditionally, BIOS has been stored on DIP ICs which can be easily removed and replaced as needed. Such ICs starts with a "27" prefix, them have two or three digits indicating the number fo kilobits (divide by 8 for kilobytes). For example, the 2764 is a ROM providing 8 kbytes of storage. If flash BIOS is used, you will find the BIOS ROM in a PLCC holder.  
• CMOS / RTC. This is a dual-function IC which maintains system setup variables in up to 128 bytes of low-power CMOS RAM. It also supplies a real-time clock which keeps track of the date, day and time. Traditional PCs used the Motorola MC146818, but many other variations have come into use. You can often locate the CMOS/RTC IC by its large, rectangular shape. IT will also locate the COMS/RTC IC by its large, rectangular shape. It will also be located near the system backup battery.
• Clocks. Proper synchronization and signaling of the motherboard requires the use of precision oscillating ICs. There are typically two clock ICs, one for the 14.318-MHz OSC signal on the ISA bus, and one driving the CPU clock (and other processing ICs). The CPU clock IC accepts the Power Good signal from the powersupply and generates the system Reset signal. The OSC clock IC also produces a 1.19-MHz signal for the PIT.
•  PIT (programmable interval timer). The interval timer provides three channels for the PC, Channel 0 is set to produce an interrupt every 54.94 ms (the RTC time base). Channel 1 is programmed to produce an interrupt every 15.12 us to signal the start of a memory refresh cycle, which must be performed at least once every few milliseconds. Channel 2 is a noncritical timer which serves to support speaker signals.
• DMAC (direct memory access controllers). DMA is critical to operation of an XT or AT system Normally, the CPU must handle each bytes of data transferred in the system-a slow process when large volimes of data must be transferred. DMA allows data transfer without the intervention or the CPU. A single DMA controller provides four channels (an AT uses two DMACs to provide eight channels).
• DMA page registers. These are relatively simple buffers that point to the 64-kbyte area (page) that DMA transfers will take place from (or to). Early PCs used one page register, but i386 and later systems often use two. Remember that is a common for newer systems to indicate a fault in the second page register because the page registers are usually integrated into chipsets.
• PIC (programmable interrupt controller). The PIC recognizes and prioritizes hardware interrupts, then passes the interrupt signal to the CPU along with a vector which points to the location of the interrupt handler routine. The XT used one PIC which supplied eight channels, but ATs use two PICs for sixteen channels (IRQ o-15).
• KBC (keyboard controller). The KBC is a very specialized single-chip microcontroller (including its own small amount of BIOS) designed as an interface between the system and the AT serial keyboard. On system initialization, the POST will cause a KBC self-test. If the self-test passes, the system can proceed to test the keyboard. Another vital part of the keyboard controller is that it handles the A20 gate. The A20 gate handles access to extended/ expanded memory (protected-mode addressing). If the KBC fails, the system will be unable to work in the protected mode.
• Gate A20. This is the simple gate that controls address line A20, and thus controls protected-mode operation by accessing memory over 1 Mbyte. The A20 gate is operated from the KBC.
• BUS controller. This device accepts control and timing signals from the system and generates the I/O and memory read and write signals needed to transfer data among system components also manages the translation of 16-bit words into 8-bit words for single-byte data transfers, and then the translation back again.
• UART (universal asynchronous receiver/transmitter). UTARTs are the key components in seral communications ports. They translate parallel data into framed serial data for transmission, and reverse the process to convert framed serial data back into parallel data during reception. Current systems use 16550A UTRATs. When a serialport error is reported, the UART has probably failed. UARTs incorporated in the motherboard are typically provided with jumpers that will set the port I/O address and IRQ line, and allow you to disable the UART in favour of expansion board communication devices.
•  DRAM. General storage and main system memory is priovided by dynamic RAM. Modern DRAM ICs can hold a great deal of data, but it must be refreshed every few milliseconds-otherwise, the data will be lost. A key purpose of the DMAC and PIC is to manage refresh. Some DRAM is fabricated on the motherboard, but much more DRAM can be added in the form of SIMMs.
•  Cache. Cache has become an important element of PC performance improvement. If data and instructions are loaded into fast static RAM in advance of the CPU's need, memory wait states can be eliminated. Many systems are equipped with 128 to 256 kbytes of externated. Many systems are equipped with 128 to 256 kbytes of external cache, but some systems can have up to 512 kbytes of cache. To boost performance every further, newer CPUs are designed with a small amount of internal cache. If external chane fails, you may be able to circumvent teh error by disabling the cache through the CMOS setup. If internal cache fails, the CPU will have to be replaced.