# VP44 Protocol Reference

Quick reference for hardware protocol constants used in HC_APTBS. Consult this before modifying any protocol-related code.

---

## DFI encoding (EEPROM address 0x0044 on VP44 ECU)

**Read (raw byte → DFI value):**
```csharp
dfi = (sbyte)raw * 3.0 / 256.0
```

**Write (DFI value → raw byte):**
```csharp
raw = (sbyte)((dfi * 256.0f) / 3.0f)
// If raw == 0, use 1 — the ECU rejects zero
```

**Checksum:**
```csharp
checksum = (byte)(0 - (byte)rawValue)
```

---

## IIR low-pass filter (PcanAdapter.PassFilterUpdate)

```csharp
result = Math.Round(prev + alpha * (value - prev), 4)
```

Used to smooth CAN bus sensor readings. The `alpha` coefficient controls responsiveness vs. noise rejection.

---

## OEM legitimation (PcanAdapter.Connect)

```csharp
token = (20120378UL << 32) | 21200UL
```

Sent as a CAN frame to authenticate with the bench controller. Without this, the bench ignores all commands.

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## K-Line / KWP timing

**Inter-byte delay** (`KW1281Connection.SendPacket`):
```csharp
Thread.Sleep(5)  // after each byte — KWP protocol requirement, NOT a workaround
```

**5-baud slow-init** (`KwpCommon.WakeUp`):
- Bit-bangs the ECU address at 5 baud on the K-Line
- Sleep durations in WakeUp are protocol-mandated — do not change

**Session lifetime**:
- One `FtdiInterface` open/close per KWP operation
- ECU expects a fresh session per dialog — do not pool connections

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## CAN bus (PCAN-Basic, 500 kbps)

- Channel: `PCAN_USBBUS1`
- Baud: `PCAN_BAUD_500K`
- Read thread polls with `Thread.Sleep(2)` in `DrainMessageQueue` (known debt)

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## Caution

Do not change any of these values without understanding the hardware protocol specification. Incorrect values can cause ECU communication failures or bench malfunction.