The Microchip 93C66BT-I/MC is a 4Kbit non-volatile EEPROM that talks over a Microwire serial interface — a three-wire bus (clock, data in, data out) that predates I2C and SPI but still shows up in industrial sensors, legacy PLC I/O modules, and calibration-storage slots where the host controller has a dedicated Microwire port. Organized as 256 x 16 bits, it clocks up to 2 MHz and writes a word or page in 2 ms.
Industrial temp range — field-swap friendly
If you are replacing one in a panel on a cold morning, the part will work at temperature — no warm-up required. The 8-DFN package (2x3 mm) with exposed pad needs a hot-air station for rework; it is not socket-friendly, but with a fine tip and some flux it is a field-swap you can do without a full reflow oven.
The base product number is 93C66B; the -I/MC suffix flags the industrial temp range and the 8-DFN package.