HX21S Logic Clarification - The Real Meaning

HX21S Constant Analysis

HX21S = 0x000E (octal 000016) = bits 1, 2, 3

From the C# ReceiverStatusBits enum: - Bit 1: StatusAvailable - Status information available in Receiver Status Register - Bit 2: ReceiverActive - Receiver is active within a frame
- Bit 3: SyncFlagReceived - At least one SYNC character or FLAG received

SINTRAN Logic Re-Analysis

Context 1: Inside X.21 Error Handler (Line 104462)

IF A/\ HX21M >< 0 THEN                    % IF X.21 error detected (bits 13-14)
   % ... handle X.21 error ...
   IF A BIT HX21S THEN                    % IF any of bits 1,2,3 are set
      HASTAT BONE BLDON=:HASTAT           % Set BLOCK DONE  
      LIINT.LKEY BONE XBLDN=:X.LKEY       % Terminate processing
   FI
FI

Context 2: Frame Processing Logic (Line 104641)

IF A /\ "LMASK" = 3 THEN                  % If frame type/length = 3
   A:=0; CALL SCRET; CALL SADTS           % Clear and return
ELSE
   IF A BIT HX21S THEN EX21 ELSE EINP FI  % If bits 1,2,3 set: EX21, else EINP
   CALL SCRET
   % ... continue processing ...
FI

The Real Logic

HX21S is NOT X.21 Clear Indication Bit!

HX21S = 0x000E tests for active receiver state bits: - StatusAvailable (Bit 1): Status info ready - ReceiverActive (Bit 2): Currently receiving frame - SyncFlagReceived (Bit 3): SYNC/FLAG detected

When HX21S Test Triggers:

1. Inside X.21 Error Handler:

   if ((rrts & 0x6000) != 0)  // X.21 error detected (bits 13-14)
   {
       if ((rrts & 0x000E) != 0)  // If receiver is active (bits 1,2,3)
       {
           setBlockDone();  // Terminate current frame processing
       }
   }
   

   Meaning: If X.21 error occurs AND receiver is active, terminate the current frame.

2. In Frame Processing:

   if ((frameType & LMASK) == 3)  // Special frame type
   {
       clearAndReturn();
   }
   else
   {
       if ((rrts & 0x000E) != 0)  // If receiver active/status available
       {
           callEX21();  // Handle active receiver
       }
       else
       {
           callEINP();  // Handle inactive receiver  
       }
   }
   

   Meaning: Route processing based on receiver activity state.

Correct Interpretation for Your HDLC Emulation

For Normal LAPB Packet Processing:

You should set these bits in RRTS when packet arrives:

1. Bit 0 (DataAvailable) = 1 - Character/packet ready for read
2. Bit 1 (StatusAvailable) = 1 - Status information available
3. Bit 2 (ReceiverActive) = 0 - Not currently within frame (packet complete)
4. Bit 3 (SyncFlagReceived) = 1 - SYNC/FLAG detected (frame boundaries)
5. Bits 13-14 (X21D/X21S) = 0 - No X.21 protocol errors

HX21S Check Result:

// HX21S = 0x000E tests bits 1,2,3
// With above settings: StatusAvailable=1, ReceiverActive=0, SyncFlagReceived=1  
// Result: (rrts & 0x000E) = 0x000A (bits 1,3 set) != 0

if (x21ErrorDetected)  // Only if bits 13-14 set
{
    if ((rrts & 0x000E) != 0)  // Receiver has activity
    {
        terminateCurrentFrame();  // Clean termination
    }
}

// For normal processing (no X.21 errors):
if ((rrts & 0x000E) != 0)  // Receiver active/status available
{
    processActiveReceiver();   // EX21 path - normal packet handling
}
else  
{
    processInactiveReceiver(); // EINP path - error/idle handling
}

Recommendations for Your HDLC Emulation

Normal Packet Reception RRTS Value:

ReceiverStatusBits rrts = 
    ReceiverStatusBits.DataAvailable |      // Bit 0 = 1 (packet ready)
    ReceiverStatusBits.StatusAvailable |    // Bit 1 = 1 (status ready) 
    ReceiverStatusBits.SyncFlagReceived;    // Bit 3 = 1 (frame sync)
    // Bit 2 (ReceiverActive) = 0 (frame complete)
    // Bits 13-14 (X21D/X21S) = 0 (no X.21 errors)

// Result: 0x000B = bits 0,1,3 set
// HX21S test: (0x000B & 0x000E) = 0x000A != 0 (will take EX21 path)

Error Condition RRTS Value:

ReceiverStatusBits rrts = 0x0001;  // Only DataAvailable, no status/sync
// HX21S test: (0x0001 & 0x000E) = 0x0000 == 0 (will take EINP path)

The HX21S logic is correct - it routes between active receiver processing (EX21) and error/inactive processing (EINP) based on receiver state bits, not X.21 clear indication!