Unusual silicon-on-sapphire chip unearthed in ancient HP floppy disk drive

In a nutshell: Ken Shirriff is an IC opposite engineering enthusiast who enjoys restoring antique computer systems and devices. While repairing an 8-inch HP floppy pressure, the laptop historian determined an obsolete production generation that became almost lost to time.

The antique floppy power had a damaged interface chip, Shirriff defined on his blog. He determined to decap it and took pictures. The chip has an uncommon substrate, which include a sapphire base with silicon factors and steel wiring on top. The "silicon-on-sapphire" chip is in part transparent, Shirriff notes, and changed into designed to characteristic as an interface among HP's interface bus (HP-IB) and the Z80 processor, which acts because the important hub inside the floppy power controller.

The silicon-on-sapphire chip is a PHI (Processor-to-HP-IB Interface) factor, utilized in various HP merchandise to manage the bus protocol and buffered statistics among the interface bus and a tool's microprocessor. The sapphire substrate imparts the chip with particular abilties, which Shirriff elaborates on in his put up.

Unlike a normal integrated circuit, the transistors on the chip are absolutely isolated due to the fact the sapphire substrate serves as an insulator. This means there is decreased capacitance between transistors, improving overall performance and imparting safety towards radiation or low-impedance short circuits.

Thanks to its natural hardening skills towards radiation, the silicon-on-sapphire configuration have been used considering the fact that 1963 or earlier, such as in spacecraft just like the Galileo probe. Shirriff in comparison the PHI chip to different processors from the 70s, which were made from each silicon-on-sapphire and silicon-most effective substrates.

HP's MC2 16-bit processor from 1977 utilized silicon-on-sapphire generation and had 10,000 transistors, in step with the historian, running at 8 megahertz on simply 350 mW of electricity. In contrast, the Intel 8086 16-bit CPU from 1978 was applied on a "regular" silicon substrate the usage of NMOS instead of the CMOS manufacturing manner. The chip had 29,000 transistors, ran at 5 megahertz first of all, and ate up up to 2.5 watts of energy.

While silicon-on-sapphire substrates provided a few advantages in terms of overall performance and electricity intake, as Shirriff's examination confirms, they weren't as densely filled with transistors as silicon ones. Furthermore, "crystal incompatibilities" between silicon and sapphire made production tough, resulting in a nine percentage yield for HP. This issue in all likelihood played a sizeable function in setting up silicon because the cloth of desire for IC production in next years.