INET Register Protocol

Overview

INetActor is the bridge between the Akka.NET actor system and the external simulator modules (MCDU displays, Navigation Displays, FCU) that communicate over UDP broadcast via the ASTLibrary BroadcastRcv1 and INetClient1 WinForms controls on frmMain. The FMGC identifies itself to the network as module ID 55, set via INetClient1.SetModuleID(55, "FMGC") in frmMain_Load before the actor system starts. All INET messages are integer register numbers with string value payloads; the protocol is 1:1 compatible with the original VB6 implementation.

Architecture

Inbound Path

Inbound registers arrive as UDP broadcasts from external modules and flow through the following steps:

An external module (FCU, MCDU display, ND, flight simulator) sends a register value via UDP broadcast.
The BroadcastRcv1 or INetClient1 ASTLibrary control fires its DataRx event on the Windows UI thread.
frmMain.INetClient1_DataRx(cmd As Integer, data As String) — the event handler — calls _inetActor.Tell(New RegisterReceived(cmd, data)), crossing from the UI thread into the actor system.
INetActor.HandleRegisterReceived publishes the RegisterReceived message to Context.System.EventStream.
Subscriber actors (KernelActor, McduActors) receive the event via their EventStream subscriptions, registered in PreStart.

Outbound Path

Outbound registers are initiated by actors and marshalled back to the UI thread for transmission:

An actor (typically GuidanceActor, McduActor, or AcarsActor) calls _inetActor.Tell(New RegisterSend(registerNumber, targetModule, value)).
INetActor.HandleRegisterSend checks whether _frmMain is available. If not (e.g., during startup before InitINet arrives), the message is queued in _pendingQueue.
Once _frmMain is available, INetActor calls frmMain.BeginInvoke(New Action(...)) to marshal the send to the UI thread.
On the UI thread: _frmMain.INetClient1.DataTX(msg.RegisterNumber, CByte(msg.TargetModule), msg.Value) transmits the register.
Note: The second parameter of DataTX is Byte, not Integer — the CByte cast is required; omitting it causes a compile or runtime error.

Startup Queue Drain

Sends that arrive before InitINet (which delivers the frmMain reference) are queued in _pendingQueue As Queue(Of RegisterSend). When InitINet arrives, INetActor drains the queue in FIFO order before processing new messages.

CreateRoute Register Subscription

INetActor supports CallCreateRoute and CallCreateRoutes messages that marshal INetClient1.CREATEROUTE(dataLink, moduleId) calls to the UI thread via BeginInvoke. CreateRoute tells ASTServer which registers the FMGC module wants to receive. ASTServer sends register data in CreateRoute request order, so the ordering of calls determines which data arrives first.

Two-Phase 8999 Handshake

Register 8999 is the routing-complete signal. The startup uses a two-phase handshake matching VB6 frmMain.frm:

After NavDb preload completes, FmgcSupervisor sends CallCreateRoute("8999") — first subscribe (VB6 line 4067)
ASTServer echoes 8999 back — FmgcSupervisor fires SEQ 5 ("SERVER DATA RECEIVING") and sends ~97 bulk CallCreateRoutes in exact VB6 order (lines 5165-5293), with "8999" as the last entry
ASTServer processes all CreateRoutes and echoes 8999 again — FmgcSupervisor fires SEQ 6, sends merged OPCAMI+AIRAC StatusSnapshot, MCDUs auto-navigate to STATUS

Because 8999 is the last CreateRoute in the bulk sequence, receiving it guarantees all other registers have been routed. MCDU keypresses are discarded during this entire sequence (while on the Startup page).

Inbound Register Table

All 18 registers cached by KernelActor for AircraftState assembly, plus 6 MCDU input registers consumed by McduActors.

Aircraft State Registers (Consumer: KernelActor)

Register	Scaling	Parameter	Notes
1703	raw / 10000	Latitude (decimal degrees)	Also forwarded via KernelStateSnapshot → McduContext for GPS Monitor page
1704	raw / 10000	Longitude (decimal degrees)	Also forwarded via KernelStateSnapshot → McduContext for GPS Monitor page
1768	raw / 10	True track (degrees)	Also forwarded to GPS Monitor page (TTRK field)
1706	raw / 10	True heading (degrees)	Magnetic heading derived: TrueHeading − MagVariation
1705	raw / 10	Magnetic variation (degrees)	Re-derives MagHeading on each update
1025	raw	True airspeed (knots)
1766	raw	Ground speed (knots)	Also forwarded to GPS Monitor page (GS field)
1099	raw	Pressure altitude (feet)
1009	raw	Barometric altitude (feet)	Also forwarded to GPS Monitor page (GPS ALT field)
1718	raw / 10	Wind direction (degrees true)
1717	raw	Wind speed (knots)
2345	raw	Flap lever position (0–4)
2381	raw	Thrust detent position
1001	1 = on ground	On-ground flag	Boolean: CInt(raw) = 1
2284	bit 16	FCU managed heading mode	Boolean: (flags And 16) = 16
1082	raw	Gross weight (kg)
1071	raw	OAT (°C)

MCDU Input Registers (Consumer: McduActors)

Register	Parameter	Side
2258	CPT scratchpad input	Captain
2260	CPT keystroke (LSK: 1000–1011, hardkeys: other values)	Captain
2254	CPT CLR key	Captain
2357	FO scratchpad input	First Officer
2358	FO keystroke (LSK: 1000–1011, hardkeys: other values)	First Officer
2353	FO CLR key	First Officer
2731	EPR mode (1=EPR, 0=N1)	Both — processed before startup guard; updates `_eprMode` in McduActor; re-renders SCREENING page if active

Outbound Registers

Registers sent by GuidanceActor and other actors to external simulator modules:

Register	Parameter	Source Actor	Notes
2240	Cross-track error (XTE × 10, signed)	GuidanceActor	Send-on-change
2288	Heading to FCU (degrees magnetic)	GuidanceActor	Send-on-change; wind-corrected
2293	Commanded managed speed (knots)	GuidanceActor	Send-on-change
2307	Mach mode target (Mach × 1000)	GuidanceActor
2334	Reference speed (IAS for current phase)	GuidanceActor
2344	Mach mode flag (0 or 1)	GuidanceActor
2331	Approach phase active flag (0 or 1)	GuidanceActor	Set on DECEL passage
2539	FMS NAV allowed flag (1 = valid)	GuidanceActor
5339	Multiplex: VOR/NDB autotune + MCDU screen text + scratchpad + ADEP/ADES	GuidanceActor, INetActor, McduActor	Autotune: "02446FV1{freq}"/"02446FA1{freq}". Screen: "02631"/"02632" + lstMCDU payload (CPT/FO). Heartbeat: "02637"/"02638" + counter. Scratchpad: "02257" (CPT) / "02356" (FO) + text (clear="a"). ADEP/ADES: "02706" + "EDDF/LPMA"
5111	ILS autotune frequency and course	GuidanceActor (RadioNavModule)	Approach phase only; BCD format
2605	Autotune bits bitmask	GuidanceActor (RadioNavModule)
5340	Multiplex register: ident, track, dist, UTC	GuidanceActor	Sequential sub-address multiplexing (GUID-06)
5535	ACARS printer output	AcarsActor	Print ACARS messages
5601	GSX menu trigger	McduActor (AocGroundServicePage)	D-14

Screen Buffer Output

MCDU screen content is sent via two parallel paths:

Path 1 — ARINC 429 binary (hardware MCDU displays): : Each McduActor owns an Arinc429ScreenBuffer instance, builds a differential binary message via BuildDifferentialMessage, and sends it as a SendScreenBuffer message to INetActor. INetActor delegates to Arinc429UdpSender.SendMessage, which applies XOR scrambling and sends the payload via UdpClient to 255.255.255.255:65520 as a raw binary datagram (register 7100 equivalent).

Path 2 — INET register 5339 text (software MCDU display / ASTServer): : After rendering, BuildLstMcduLines() reconstructs VB6-format text lines from the binary buffer. INetActor joins them with Chr(26) and sends via INetClient1.DataTX 5339, 0, prefix & payload. Sub-address prefixes: "02631" (CPT screen), "02632" (FO screen), "02637" (CPT heartbeat), "02638" (FO heartbeat). The A320_MCDU software display app subscribes to CMD 2631/2632/2637/2638 to render pages.

Path 3 — Scratchpad text (software MCDU display): : McduActor.SendScratchpad() sends the current scratchpad text to the external MCDU display via register 5339. Sub-address prefixes: "02257" (CPT), "02356" (FO). A clear command (prefix & "a") is sent first, then the content string. This matches VB6 PageHandle.cls SendScratchpad.

See ARINC 429 Screen Buffer Pipeline for the full pipeline.

Message Contracts

All INET actor messages are defined in A320_FMGC/Inet/INetMessages.vb:

Message Class	Constructor Parameters	Direction	Purpose
`InitINet`	`formMain As frmMain`	Supervisor → INetActor	Delivers frmMain reference after actor creation; triggers queue drain
`RegisterSend`	`registerNumber As Integer, targetModule As Integer, value As String`	Any actor → INetActor	Outbound register send; value must be pre-formatted with InvariantCulture
`RegisterReceived`	`command As Integer, data As String`	frmMain → INetActor → EventStream	Inbound register from DataRx; published to EventStream for subscribers
`SendScreenBuffer`	`isCaptain As Boolean, screenData As Byte(), lstMcduLines As String()`	McduActor → INetActor	Dual-path screen output: ARINC 429 binary via Arinc429UdpSender + INET register 5339 text via INetClient1.DataTX

Value Formatting

All values in RegisterSend must be formatted with CultureInfo.InvariantCulture by the sending actor before construction. INetActor passes the string directly to DataTX without transformation.

Source Files

A320_FMGC/Inet/INetActor.vb — inbound/outbound handler, BeginInvoke marshalling, queue drain, PostStop cleanup
A320_FMGC/Inet/INetMessages.vb — InitINet, RegisterSend, RegisterReceived, SendScreenBuffer
A320_FMGC/Kernel/KernelActor.vb — 18-register cache, AircraftState assembly, EventStream subscription
A320_FMGC/Mcdu/McduActor.vb — MCDU register constants (2258, 2260, 2254, 2357, 2358, 2353), scratchpad/keystroke handling
A320_FMGC/frmMain.vb — SetModuleID(55, "FMGC"), INetClient1_DataRx handler, BeginInvoke target