HDLC Critical Constants Analysis

Constants Found in SYMBOL-1-LIST.SYMB.TXT

Constant	Octal Value	Hex Value	Binary	Bit Position	Description
TXUND	000002	0x0002	0000000000000010	Bit 1	Transmitter Underrun
EMTY	004000	0x0800	0000100000000000	Bit 11	Empty Buffer/List
ERB	001000	0x0200	0000001000000000	Bit 9	Error Block
EUND	000102	0x0042	0000000001000010	Bits 1,6	Underrun Error Code
BLDON	000010	0x0008	0000000000001000	Bit 3	Block Done
SILFO	100000	0x8000	1000000000000000	Bit 15	Signal Flag Output
HX21M	060000	0x6000	0110000000000000	Bits 13-14	X.21 Error Mask
HX21S	000016	0x000E	0000000000001110	Bits 1-3	X.21 Status
XBLDN	000010	0x0008	0000000000001000	Bit 3	External Block Done

Critical Analysis - The Retransmission Bug!

SILFO+TXUND Mask Analysis

IF A/\ "SILFO+TXUND" = 0 THEN

SILFO+TXUND = SILFO | TXUND = 0x8000 | 0x0002 = 0x8002

This means the condition tests: - SILFO (Bit 15) = 0x8000 - Signal Flag Output - TXUND (Bit 1) = 0x0002 - Transmitter Underrun

🚨 MAJOR DISCOVERY - Logic Error Found!

// Current SINTRAN logic:
if ((hastat & 0x8002) == 0) 
{
    // SUCCESS PATH - both SILFO and TXUND are CLEAR
    clearRetryState();
    logSuccess();
}
else 
{
    // ERROR PATH - either SILFO or TXUND is SET
    // This causes RETRANSMISSION!
    logError();
    retransmitPacket();
}

The Problem: - SILFO (Bit 15) appears to be a status flag, not an error flag - TXUND (Bit 1) matches TransmitterUnderrun from the C# enum - The logic treats ANY set bit as an error condition

Comparison with C# Enum:

• TXUND (0x0002) = TransmitterUnderrun (Bit 1) ✅ Correct
• SILFO (0x8000) = Illegal (Bit 15) from TransmitterStatusBits ✅ This IS an error!

Receiver Constants Analysis

EMTY (Empty Buffer)

• EMTY = 0x0800 = Bit 11 matches ListEmpty from ReceiverStatusBits ✅
• Used in: IF HASTAT/\"EMTY" >< 0 THEN - stops receiver if no buffers

X.21 Protocol Constants

• HX21M = 0x6000 = Bits 13-14 matches X21D | X21S ✅
• HX21S = 0x000E = Bits 1-3 ❌ MISMATCH!

X.21 Issue:

// From C# enum: X21S should be bit 14 = 0x4000
// From constants: HX21S = 0x000E = bits 1-3
// This is a MAJOR discrepancy!

Block Control Constants

• BLDON = 0x0008 = Bit 3 - Block done flag
• XBLDN = 0x0008 = Bit 3 - Same as BLDON (likely different context)

Root Cause Analysis

1. Transmission Success Logic ✅ CORRECT

The SILFO+TXUND logic is actually CORRECT:

// SILFO (0x8000) = Illegal format/key error (Bit 15)
// TXUND (0x0002) = Transmitter underrun (Bit 1)
// Both are ERROR conditions

if ((rtts & 0x8002) == 0) 
{
    // SUCCESS: No illegal format AND no underrun
    success();
}
else 
{
    // ERROR: Either illegal format OR underrun
    retransmit();
}

2. X.21 Status Mismatch ❌ POTENTIAL BUG

// Expected: HX21S = X21S (bit 14) = 0x4000
// Actual:   HX21S = 0x000E (bits 1-3)

// Current logic:
if ((rrts & 0x000E) != 0) // Tests bits 1-3
{
    setBlockDone(); // Wrong bits being tested!
}

// Should be:
if ((rrts & 0x4000) != 0) // Test bit 14 (X21S)
{
    setBlockDone();
}

Updated Bit Usage Analysis

RTTS (Transmitter) - CORRECT Logic

ushort rtts = ReadRTTS();

// Check for errors (CORRECT)
if ((rtts & 0x8002) != 0) // SILFO (Illegal) OR TXUND (Underrun)
{
    retransmitPacket(); // Correct response to errors
}
else
{
    transmissionSuccess(); // No errors detected
}

RRTS (Receiver) - X.21 Logic Issue

ushort rrts = ReadRRTS();

// X.21 error check (POTENTIALLY WRONG)
if ((rrts & 0x6000) != 0) // HX21M - checks bits 13-14 ✅ CORRECT
{
    handleX21Error();

    // X.21 clear indication check (WRONG BITS)
    if ((rrts & 0x000E) != 0) // HX21S - should be 0x4000 (bit 14)
    {
        setBlockDone(); // May trigger incorrectly
    }
}

Recommendations

1. Verify X.21 Clear Indication Logic - HX21S constant seems wrong
2. SILFO+TXUND logic is actually CORRECT - both are error conditions
3. Check if HX21S should be 0x4000 instead of 0x000E
4. The retransmission issue may be elsewhere - not in the SILFO+TXUND logic

The transmission logic appears correct based on these constants. The issue might be in the X.21 status interpretation or timing of status reads.