The OBD1 Honda ECU platform (such as the P28, P30, and P72) is built around the OKI 66207 microcontroller (66k instruction set). To manage the diverse inputs and outputs required to run an engine, the MCU utilizes a mix of direct pins, analog multiplexers, and a custom 82C55 I/O expander.

1. Hardware I/O Architecture#

The ECU classifies signals based on their timing priority and signal type:

• Direct MCU I/O: High-priority or time-sensitive signals are connected directly to the MCU. These include engine speed sensors (CYP, CKP, TDC), vehicle speed sensor (VSS), and injector driver control circuits.
• Analog Inputs: Sensor voltages (MAP, TPS, IAT, ECT, Oxygen sensor) are routed directly to the MCU's internal analog-to-digital converter (ADC) via multiplexer ICs.
• 82C55 I/O Expander: Most digital switches (clutch switch, A/C request) and low-priority solenoid outputs (VTEC solenoid, A/C clutch relay, EVAP purge) are managed by a custom 44-pin Intel 82C55-compatible I/O expander chip.
• Input Latch: A 20-pin logic latch IC located near the EPROM acts as an additional 8-bit digital input buffer for chassis and transmission switches.

2. Memory-Mapped Registers and RAM Buffers#

Because the 82C55 is mapped as a peripheral device in the ECU's memory space, it does not have direct RAM addresses. Instead, the software uses specific RAM locations as intermediate input/output buffers. During execution, the ECU continuously synchronizes these RAM buffers with the physical hardware registers (mirrored at memory spaces 0x0F00, 0x1F00, and 0x2F00).

Common I/O Buffer Locations (P30 / P28 Codebases)#

3. Assembly Code Analysis (OKI 66k)#

The following examples demonstrate how the ECU processes RAM buffers using the JDM OBD1 P30 (P30_203) codebase.

Example A: A/C Clutch Control Logic#

This routine checks the state of the A/C cabin button and controls the compressor clutch relay output:

4306- DA 11 17 : JBR off 11.2, 4320 ; Jump to 4320 if A/C cabin switch (11.2) is OFF
4309- C4 26 1C : SB off 26.4 ; Set bit 26.4
430C- F4 E2 : LB A, off E2
430E- CE 1B : JNE 432B
4310- C4 E3 98 32 : MOVB off E3, #32
4314- C4 1B 18 : SB off 1B.0
4317- 95 : RC
4318- CB 15 : SJ 432F
431A- C4 B8 15 : CLRB off B8
431D- C4 E3 15 : CLRB off E3

4320- C4 26 0C : RB off 26.4 ; Reset bit 26.4
4323- F4 E3 : LB A, off E3
4325- CE ED : JNE 4314
4327- C4 E2 98 32 : MOVB off E2, #32
432B- C4 1B 08 : RB off 1B.0
432E- 85 : SC
432F- C5 20 38 : MB 20.0, C ; Output state to A/C Clutch Solenoid (20.0)

Example B: VTEC Pressure Switch (VTS) Feedback#

To prevent VTEC activation if oil pressure is insufficient, the ECU compares the VTEC solenoid output state against the VTS sensor input.

3D06- DC 16 1B : JBR off 16.4, 3D24 ; Jump if VTEC is software-disabled
3D09- EC 14 18 : JBS off 14.4, 3D24 ; Jump if VTS error flag (14.4) is active
3D0C- DA 1F 0C : JBR off 1F.2, 3D1B ; Jump if VTEC solenoid output is not active
3D0F- C4 C0 98 14 : MOVB off C0, #14
3D13- F4 BF : LB A, off BF
3D15- ED 10 0C : JBS off 10.5, 3D24 ; Jump if VTS (10.5) is active
3D18- 85 : SC
3D19- CB 09 : SJ 3D24
3D1B- C4 BF 98 14 : MOVB off BF, #14
3D1F- F4 C0 : LB A, off C0
3D21- ED 10 F4 : JBS off 10.5, 3D18
3D24- C5 9A 38 : MB 9A.0, C ; If mismatch, set bit 9A.0 to trigger Code 21

Tip

Tuners can patch the check on bit 10.5 to allow VTEC operation on engines lacking the physical VTEC oil pressure switch, common in JDM cylinder head swaps.

4. Hardware Jumpers (RP17, RP18)#

The ECU's configuration jumpers (RP17 and RP18) are connected directly to multiplexed analog inputs. These jumpers act as resistance dividers; the ADC reads the resulting voltage to determine the chassis configuration, regional market, or transmission type (automatic vs. manual) the ECU software should assume.