Matt Iselin Brings the I2C Bus to Vintage Computers' Eight-Bit ISA Slots for Flexible Expansion
For the cost of one slot, you could add a range of hardware to your classic IBM compatible — from displays to temperature sensors.
Software engineer and vintage computing enthusiast Matt Iselin has designed an add-in board that brings the benefits of an easily-accessible I2C bus connection to any computer that can accept an eight-bit ISA card.
"The ISA-to-I2C Interface Card allows systems to access peripherals on an I2C bus," Iselin explains. "The card is based around the [NXP Semiconductors] PCF8584 IC, and can operate at a frequency of up to 90kHz when operating as the I2C bus controller. While the PCF8584 is an older part, it integrates better with the 5V-centric ISA bus. The card uses hand-solderable through-hole components for all components except the PCF8584 chip, which is surface-mount to reduce the overall cost of the card."
Released in 1981 as the IBM PC/XT bus before becoming known as the Industry Standard Architecture bus, the eight- and 16-bit ISA slot was a familiar sight in desktop PCs — and even some chunky portables — until the 32-bit Peripheral Component Interconnect (PCI) took over. While the ISA slot still sees some limited use industrial equipment, Iselin's interest is more in line with its original use in desktop PCs — which is where his add-in board comes in.
"The card can be used to add additional functionality to a system without consuming ISA card slots," Iselin explains of why you'd want your vintage desktop to get a dedicated I2C bus connection. "For example, you could add a real-time clock, temperature sensor, and EEPROM, all for the cost of a single ISA card slot. To simplify connectivity to external I2C devices, STEMMA connectors are used. These provide +5V and GND, in addition to the I2C SDA and SCL signals. A great deal of external modules are available that can be connected through these connectors." A DE-9 connector, meanwhile, provides connectivity for devices connected from outside the PC's chassis.
Iselin has released KiCad project files for the board under the permissive variant of the CERN Open Hardware License Version 2 on GitHub, as part of the broader M88 Computer project; assembled cards are up for sale on Iselin's Tindie store at $64.99.