Feature

Milestones That Mattered: CMOS pioneer developed a precursor to the processor

By Maury Wright, Editor in Chief -- EDN, 6/22/2006

From EDN, April 1, 1970 COS-MOS Could Put Computer Slice on a Chip PHILADELPHIA - One direction in which LSI may take off in the next few years was dramatically illustrated at ISSCC by a description of a complete 4-bit slice of a computer's arithmetic section built for NASA by RCA. The 775-transistor COS-MOS (Complementary Symmetry MOS) LSI chip described by Allan Alaspa and Andrew Dingwall of RCA impressed experts at ISSCC because it showed that it is now feasible to cram all the control and temporary storage registers and arithmetic logic needed for a 4-bit slice of a parallel arithmetic process into a "sensible" monolithic unit. Click here for the complete text of this 1970 article.

Electronic engineers under the age of 40 or so may not even know that RCA was once a player in the semiconductor market. But RCA made a variety of semiconductors and was the leading supporter of CMOS back when those in the industry considered CMOS a slow technology that was suitable only in specialty, low-power applications. RCA engineers not only designed CMOS-logic chips, but also were headed toward a microprocessor, as this account in 1970 from an early International Solid-State Circuits Conference indicates.

RCA in 1968 introduced the first CMOS ICs—the 4000 series of logic ICs. The series included functions similar to the 7400 series of TTL ICs. But CMOS, which RCA called COS-MOS at the time, offered far lower power. CMOS had other advantages, as well. The 4000 series could operate from supplies of 3 to 15V and could handle a fan-out of 50 or more devices, whereas TTL could handle a maximum of 10 devices. But early CMOS was far more susceptible to electrostatic discharge than TTL. And, when RCA introduced it, the 4000 series could operate at only 1 MHz, whereas TTL ICs could operate at 10 MHz.

The account of RCA's bit-slice development clarifies the early performance issues. (The sidebar on the right is an excerpt; click here to read the entire story and see a schematic.) The RCA engineers claimed that their copy of the PDP-8 would run at 250 kHz or perhaps 500 kHz. The early PDP-8 operated at 666 kHz.

In the early days of the microprocessor, bit-slice approaches such as the RCA example were popular. A bit-slice design could offer performance advantages in operating frequency and allowed designers to customize the instruction set.

This milestone also provides another example of how important a role NASA played in driving semiconductor developments. It's likely that NASA needed the low-power capabilities that CMOS delivered. And the semiconductor wizards that make our industry so much fun ultimately figured out how to make CMOS blazingly fast as it came to dominate first the 7400 series of logic and then processors, and it today serves even in analog and mixed-signal designs.