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ARINC 429 Screen Buffer Pipeline

Overview

The ARINC 429 screen buffer pipeline carries MCDU page content from the McduCommandParser character writer all the way through to a UDP datagram sent to port 65520 as register 7100 payload. The key design feature is a differential XOR optimization: instead of transmitting all 312 character positions on every page render, the pipeline tracks which cells have changed since the last transmission and sends only the dirty range. A typical page refresh sends 50–100 bytes instead of the full 624-byte buffer, significantly reducing UDP bandwidth to the MCDU display hardware.

Pipeline Stages

Buffer Dimensions

The screen buffer models 13 lines of 24 columns each, giving 312 total character positions:

  • LINES As Integer = 13
  • COLUMNS As Integer = 24
  • BUFFER_SIZE As Integer = 312 (= LINES × COLUMNS)

Two parallel byte arrays hold the screen state:

  • _charBuffer(311) As Byte — 312-element array (0-based). Each element is one ARINC 429 character byte.
  • _ctrlBuffer(311) As Byte — 312-element array (0-based). Each element is a control byte encoding font and color: CByte(CType(font, Byte) Or CType(color, Byte)).

Buffer index for a given position: pos = (line * 24) + column where both line and column are zero-based.

Dirty Range Tracking

Two integer fields track the range of modified positions:

  • _firstChange As Integer = Integer.MaxValue — lowest position index modified since last reset
  • _lastChange As Integer = -1 — highest position index modified since last reset

On each WriteChar or SetControl call: 

_firstChange = Math.Min(_firstChange, pos)
_lastChange  = Math.Max(_lastChange, pos)

HasChanges() returns True when _firstChange <= _lastChange.

Character Encoding

Color Codes

Defined in the McduColor enum (A320_FMGC/Inet/Arinc429Constants.vb), matching VB6 Variables_ARINC429.bas:

Color Byte Value McduCommandParser Token
Black 0 (default background, not selectable by token)
Cyan 16 ~b
Red 32 ~r
Yellow 48 ~y
Green 64 ~g
Magenta 80 ~m
Amber 96 ~a
White 112 ~w

Font Codes

Font Byte Value McduCommandParser Token
Small 0 @s
Large 128 @l

Control Byte Composition

The control byte combines font and color with bitwise OR:

_ctrlBuffer(pos) = CByte(CType(font, Byte) Or CType(color, Byte))

Example: Large (128) + White (112) = 240 (0xF0). Small (0) + Cyan (16) = 16 (0x10).

Special Character Mapping

Arinc429SpecialChars.MapSpecialChar(c As Char) As Byte maps shorthand characters used in MCDU page source strings to their ARINC 429 display byte values:

Input Char Output Byte Symbol
a 29 Square symbol
b 28 Degrees symbol
i 110 Flyover waypoint symbol
c 95 Left arrow
d 31 Right arrow
e 94 Up arrow
f 30 Down arrow
all others ASCII byte Pass-through (ASCII value AND 0xFF)

Differential XOR Transmission

BuildDifferentialMessage

BuildDifferentialMessage(isCaptain As Boolean) As Byte() produces the binary payload ready for Arinc429UdpSender.SendMessage:

  1. Returns an empty array (Array.Empty(Of Byte)()) if there are no pending changes (HasChanges() = False).
  2. Computes changeCount = _lastChange - _firstChange + 1 — the span of the dirty range.
  3. Allocates a totalLength = 18 + 2 * changeCount byte array.
  4. Writes an 18-byte fixed header (matching VB6 PageHandle.cls lines 245–256):
    • Bytes 0–1: 0x45, 0x46 ("EF")
    • Byte 2: 0x76 (118) for Captain, 0x7A (122) for FO
    • Bytes 3–13: fixed magic bytes
    • Bytes 14–15: start position as big-endian UInt16 (_firstChange)
    • Bytes 16–17: change count as big-endian UInt16
  5. Copies _ctrlBuffer[_firstChange .. _lastChange] into bytes 18 onward.
  6. Copies _charBuffer[_firstChange .. _lastChange] immediately after the control bytes.

After calling BuildDifferentialMessage, the caller invokes ResetDirtyRange() to reset _firstChange and _lastChange for the next cycle.

Bandwidth Savings

A full buffer transmission would be 18 + 2 × 312 = 642 bytes. A typical page refresh touches 25–50 contiguous positions, producing 18 + 2 × 50 = 118 bytes or fewer. Single-field updates (e.g., distance readout changing one value) send as few as 20–30 bytes.

UDP Output

Arinc429UdpSender wraps a single System.Net.Sockets.UdpClient instance targeting 255.255.255.255:65520 (UDP broadcast).

XOR Scrambling

Before transmission, SendMessage applies the VB6 mod_ARINC429.bas scrambling algorithm:

  1. Pad the raw message to a minimum of 24 bytes with 0x00.
  2. Generate 3 random key bytes: k0, k1, k2 (0–255 each).
  3. Compute k3 = k0 XOR k1 XOR k2.
  4. Build the scrambled output:
    • Index 0: copy as-is
    • Index 1: 0x4D ('M')
    • Index 6: 0x45 ('E')
    • Index 7: k0
    • Index 8: 0x45 ('E')
    • Index 9: k1
    • Index 10: 0x4D ('M')
    • Index 11: k2
    • Index 12: 0x01
    • Index 13: 0x01
    • All other indices: rawMessage(i) XOR k3

IDisposable and PostStop

Arinc429UdpSender implements IDisposable. The Close() method shuts down the UdpClient and sets a _disposed flag to prevent double-close. INetActor.PostStop calls _arinc429Sender.Close() to release the socket when the actor system shuts down:

Protected Overrides Sub PostStop()
    If _arinc429Sender IsNot Nothing Then _arinc429Sender.Close()
    _Logger.Info("INetActor stopped")
End Sub

Calls to SendMessage after disposal are silently discarded with a warning log.

McduActor Integration

McduActor owns one Arinc429ScreenBuffer instance per side (Captain or FO). The buffer is NOT shared between actors. The render cycle operates as follows:

  1. An input event (LSK press, scratchpad entry, periodic refresh) triggers page rendering.
  2. The active IPage implementation's HandleLsk method returns a McduCommand; separately, the Render method builds a List(Of String) of VB6 command strings into the screen buffer.
  3. McduCommandParser.ParseAndRender(commands, _screenBuffer) processes the command list:
    • "LISTCLEAR" — calls _screenBuffer.Clear() and resets parser color/font state to White/Large.
    • "LISTCOMPLETE" — no-op end marker.
    • Line content entries ("01..." through "13...") — parsed for column positions (|CC), font tokens (@s/@l), color tokens (~w/~b/~g/~y/~r/~a/~m), and character bytes.
  4. _screenBuffer.BuildDifferentialMessage(isCaptain) produces the ARINC 429 binary payload.
  5. _screenBuffer.LastCommands provides the raw VB6 command list cached during ParseAndRender — these are the original IPage commands (no binary-to-text reconstruction). ParseAndRender auto-appends LISTCOMPLETE if the page omitted it.
  6. Both outputs are passed to INetActor: _inetActor.Tell(New SendScreenBuffer(isCaptain, bytes, lstLines)).
  7. _screenBuffer.ResetDirtyRange() is called after the build.
  8. INetActor.HandleSendScreenBuffer sends via two paths:
    • Path 1 — ARINC 429 binary: delegates to _arinc429Sender.SendMessage(msg.ScreenData) for hardware MCDU displays.
    • Path 2 — INET register 5339 text: joins lstMcduLines with Chr(26), sends via INetClient1.DataTX 5339, 0, subPrefix & payload for software MCDU display. Also sends heartbeat on separate sub-address.

Source Files

  • A320_FMGC/Inet/Arinc429ScreenBuffer.vb — 13×24 buffer, _charBuffer/_ctrlBuffer, dirty range tracking, BuildDifferentialMessage, 18-byte header format
  • A320_FMGC/Inet/Arinc429Constants.vb — McduColor enum (Black=0, Cyan=16, Red=32, Yellow=48, Green=64, Magenta=80, Amber=96, White=112), McduFont enum (Small=0, Large=128), Arinc429SpecialChars.MapSpecialChar
  • A320_FMGC/Inet/Arinc429UdpSender.vb — UdpClient wrapper, XOR scrambling algorithm, IDisposable, Close()
  • A320_FMGC/Inet/INetActor.vb — HandleSendScreenBuffer, PostStop cleanup
  • A320_FMGC/Mcdu/McduCommandParser.vb — ParseAndRender, VB6 command string format, color/font token parsing
  • A320_FMGC/Mcdu/McduActor.vb — buffer ownership, render cycle, BuildDifferentialMessage call site