EDN -- 04.25.96 Coalition defines standard for PC-motherboard design

Out in Front: April 25, 1996

Coalition defines standard for PC-motherboard design

A group of electronics companies led by National Semiconductor, Opti Inc, PicoPower Technology, Standard Microsystems Corp (SMC), Texas Instruments, and VLSI Technology has announced the Common Architecture standard for PC-motherboard designs. The standard defines ways to move interrupt-request (IRQ)- or DMA-dependent legacy devices onto the PCI bus (see "The mighty morphin' PCI bus," pg 58). Moreover, you can mix and match core-logic, graphics, I/O, and bridge ICs developed in accordance with the standard without glue logic, regardless of the manufacturer of each IC.

The Common Architecture often eliminates the need for the ISA bus. The specification defines a scheme to serialize ISA-bus DMA and IRQ signals and transmit the signals via PCI. Solving legacy IRQ and DMA problems is significant from both logistic and performance perspectives. Logistically, for example, some current PCI-based disk controllers require a cable connection to an ISA slot just to intercept the DMA and IRQ signals. The Common Architecture eliminates such crude work-arounds that were previously necessary to move devices onto the PCI bus.

Moving peripherals to PCI, meanwhile, offers a widely recognized boost in data-transfer speeds. More subtly, moving low-speed devices, such as sound cards, to PCI also boosts system performance, because the PCI bus and processor sit idle while DMA transfers occur across the much slower ISA bus. PCI implementations more quickly handle the transfer and maintain legacy compatibility through the serial DMA/IRQ scheme.

The key to compatibility among motherboard ICs from different vendors is the use of the PCI bus to connect all of the major functional blocks on the motherboard. The scheme demands that the system controller alone handles processor-specific signals. It also eliminates the sideband signals that traditionally have carried control signals among the major motherboard functions, including the system controller, I/O, graphics, and bus-bridge ICs.

Despite what appears to be a win-win proposition, the Common Architecture could face a tough road to widespread deployment. Intel, by far the leading vendor of core-logic ICs and motherboards, has neither participated in nor commented on the effort. The company may be reluctant to endorse a scheme that will allow designers to mix and match its own motherboard ICs with those of other vendors. Moreover, the scheme could allow other core-logic and motherboard vendors to compete more effectively against Intel.

Intel, meanwhile, announced at WinHEC '96 in San Jose, CA, its own proposed changes to the PC architecture. The company announced plans to move the graphics controller from the 33-MHz PCI bus to a private local bus that is essentially a second PCI bus running at 66 MHz. Intel claims that the higher bandwidth connection is necessary for 3-D graphics controllers to access textures and Z-buffer data stored in the main-memory subsystem. Intel plans to begin next year to ship the AGP accelerated-graphics port in the company's core-logic chip set.

Supporters of the Common Architecture claim that compliant ICs and systems will appear in the second half of this year. Copies of the spec are available on the Web sites of Digital, National, SMC, and VLSI. —by Maury Wright

Intel Corp, Santa Clara, CA. (408) 987-8080.

National Semiconductor, Santa Clara, CA. (408) 721-5000.

Opti Inc, Milpitas, CA. (408) 486-8000.

PicoPower Technology, Fremont, CA. (510) 623-8300.

Standard Microsystems Corp, Hauppauge, NY. (516) 435-6000.

Texas Instruments Inc, Denver, CO. (800) 477-8924.

VLSI Technology Inc, San Jose, CA. (408) 434-3000.