The 82C55 (often found as the OKI MSM82C55A) is a Programmable Peripheral Interface (PPI) integrated circuit used extensively in Honda OBD1 Engine Control Units (ECUs). It acts as an input/output (I/O) expander, enabling the main OKI 66207 microcontroller to interface with various auxiliary inputs and output drivers that cannot be handled directly by the MCU's native I/O pins.

Almost all OBD1 Civic, Integra, and Accord ECUs share a common architectural pattern for the functions controlled by the 82C55 chip.

Configuration and Software Control#

In Honda OBD1 applications, the ECU software configures the 82C55 by writing 0x90 to its internal Control Register. According to the standard Intel/OKI 82C55 datasheet:

• Mode 0 (Basic Input/Output) is selected.
• Port A is configured as an Input port.
• Port B and Port C are configured as Output ports.

Note

The OKI MSM82C55 chip package found on Honda OBD1 boards has 42 pins, which is two more than the standard 40-pin package outlined in standard datasheets. These extra pins (pins 17 and 37) are unused and are likely internal connections or ground ties specific to OKI's custom packaging.

82C55 Pinout Mapping#

The following table details the electrical pinout mapping of the 82C55 PPI on standard Honda OBD1 boards.

Technical Notes & Hardware Quirks#

• Intake Resonator Multiplexing: On specific boards (e.g., JDM models or those utilizing specific software iterations), the Intake Resonator control circuit is doubled up. The trace routing follows: IC16 (pin 3) → Q27 (pin 2) → IC9 (pin 6).
• Diagnostic Inputs (Port A): Inputs PA4, PA5, PA6, and PA7 are dedicated loop-back diagnostic lines. The microcontroller reads these pins to verify that an output command (like VTEC engagement or O2 heater activation) has physically occurred. If the MCU commands PC0 (VTEC) to go high but does not detect the corresponding feedback on PA5, it triggers a Check Engine Light (CEL).