/motronic

ROM Decoding Reference

Use this reference for Porsche 911 Carrera 3.2 28-pin Bosch Motronic EPROM decoding. Treat offsets as ROM-image offsets unless disassembly proves processor address-space remapping.

Preferred Tooling

These commands refer to a separate local Motronic repository, not files bundled with this skill. Locate and inspect its decoder and supported flags before use; if unavailable, report the tooling gap and analyze supplied bytes only where the format is proven. For that repository, examples are:

uv run scripts/decode-motronic.py "911 chip 89 911 28pin 3.bin"
uv run scripts/decode-motronic.py --format=csv "911 chip 89 911 28pin 3.bin"
uv run scripts/decode-motronic.py --variant=all --maps=ign_wot,fuel_wot "911 chip 89 911 28pin 3.bin"

Useful flags:

Verified 1989 28-Pin ROM Identity

Known source file: 911 chip 89 911 28pin 3.bin

Bosch Map Structure Pattern

1D maps:

parameter_byte
axis_count
axis_delta_bytes[axis_count]
data_bytes[axis_count]

2D maps:

x_parameter_byte
x_axis_count
x_axis_delta_bytes[x_axis_count]
y_parameter_byte
y_axis_count
y_axis_delta_bytes[y_axis_count]
data_bytes[x_axis_count * y_axis_count]

Known parameter bytes in this ROM family:

Axis Decoding

Axes are delta-coded from the end of the byte sequence:

def decode_axis(axis_bytes, scale=1):
    value = 256 - axis_bytes[-1]
    out = [value]
    for delta in reversed(axis_bytes[:-1]):
        value -= delta
        out.append(value)
    out.reverse()
    return [v * scale for v in out]

Use scale=40 for RPM axes in this Porsche ROM family.

Primary Map Offsets

Alternate Map Offsets

Conversions

Fuel:

fuel_multiplier = raw / 128

Fuel maps are correction multipliers on calculated base pulse width. They are not direct AFR targets.

Ignition:

degree_per_count = 360 / (129 * 4)  # 0.697674...
degrees_btdc = raw * degree_per_count - 13.945

Use this conversion to match the known-correct 1989 911 3.2 WOT ignition decode. Reverify absolute timing with a timing light when engine safety matters.

Rev Limiter Evidence

For the known 1989 8 KB image:

Distinguish the validated limiter scalar at 7000 rpm from the highest WOT map axis point at 6840 rpm.

Verification Checklist

• Verify the image size and hash against the intended ROM family. The documented reference image is 8192 bytes; 28-pin packaging alone does not establish binary size or map layout.
• Record SHA1 and SHA256.
• Parse candidate structures from known offsets or directory pointers.
• Confirm expected shapes: 12x12 part-throttle fuel/ignition, 1x20 WOT fuel, 1x16 WOT ignition, 1x10 idle fuel/ignition.
• Confirm RPM axes are plausible using * 40 rpm.
• Confirm fuel values cluster around 128.
• Confirm ignition raw values cluster in expected Porsche range.
• Cross-check WOT terminal axes and report rev-limit scalar separately.
• State formulas and confidence levels in the final report.

Hard Rules

• Do not invent checksum behavior. Identify it from ROM family, XDF, disassembly, or tool output.
• Do not assume public 2 KB/4 KB XDF offsets apply directly to an 8 KB ROM without proving alignment.
• Show raw values when engineering-unit conversion is uncertain.
• Keep alternate maps separate from primary maps unless code or directory evidence proves selection logic.