Final HDLC Bug Analysis - With Actual Constants

Critical Constants (Octal → Hex → Binary)

Constant	Octal	Hex	Binary	Bit(s)	Meaning
TXUND	000002	0x0002	bit 1	TransmitterUnderrun
SILFO	100000	0x8000	bit 15	Illegal Format/Key Error
EMTY	004000	0x0800	bit 11	List Empty
HX21M	060000	0x6000	bits 13-14	X.21 Error Mask
HX21S	000016	0x000E	bits 1,2,3	X.21 Status (NOT bit 14!)
BLDON	000010	0x0008	bit 3	Block Done
XBLDN	000010	0x0008	bit 3	External Block Done
ERB	001000	0x0200	bit 9	Error Block
EUND	000102	0x0042	bits 1,6	Underrun Error Code

Transmission Logic Analysis - CORRECT!

SILFO+TXUND Check (Line 104046)

IF A/\ "SILFO+TXUND" = 0 THEN    % IF (A AND 0x8002) == 0
   XRETRY=:RTDYN; A:=0; CALL SADTS  % SUCCESS PATH
ELSE
   A:=HASTAT; CALL SADTS; CALL DRERR  % ERROR PATH  
   A:=EUND
FI

Analysis:

// SILFO+TXUND = 0x8000 | 0x0002 = 0x8002
ushort rtts = ReadRTTS();

if ((rtts & 0x8002) == 0)  // No illegal format AND no underrun
{
    // SUCCESS - transmission completed successfully
    clearRetryState();
    logSuccess();
}
else
{
    // ERROR - either illegal format OR underrun occurred
    logError();
    incrementErrorCounter();
    retransmitPacket();
}

This logic is CORRECT! - SILFO (0x8000) = Illegal format error (should cause retransmission) - TXUND (0x0002) = Transmitter underrun (should cause retransmission) - Only when BOTH are clear (0) is transmission successful

Reception Logic Analysis - MAJOR ISSUE FOUND!

X.21 Error Detection (Line 104450)

IF A/\ HX21M >< 0 THEN          % IF (A AND 0x6000) != 0
   % Handle X.21 error
   IF A BIT HX21S THEN          % IF A has ANY of bits 1,2,3 set
      HASTAT BONE BLDON=:HASTAT % Set block done flag
      LIINT.LKEY BONE XBLDN=:X.LKEY
   FI
FI

🚨 CRITICAL BUG FOUND:

// X.21 error mask check - CORRECT
if ((rrts & 0x6000) != 0)  // Check bits 13-14 for X.21 errors
{
    handleX21Error();

    // X.21 clear indication check - WRONG!
    if ((rrts & 0x000E) != 0)  // HX21S tests bits 1,2,3 (NOT bit 14!)
    {
        setBlockDone();  // Terminates connection
        return;
    }
}

The Problem: - HX21S = 0x000E tests bits 1, 2, and 3 - These bits are DataAvailable, StatusAvailable, and ReceiverActive - These are NORMAL operation bits, NOT X.21 clear indication! - Real X.21 Clear should be bit 14 (0x4000)

This means: - When normal data is available (bit 0 set), the X.21 clear logic triggers - Connection gets terminated inappropriately - Packets get dropped due to false X.21 clear indication

List Empty Check (Line 104463)

IF HASTAT/\"EMTY" >< 0 THEN     % IF (HASTAT AND 0x0800) != 0
   0=:ACTSW                     % Stop device
   MIN STPCNT                   % Increment stop counter

Analysis:

if ((rrts & 0x0800) != 0)  // List Empty bit set
{
    stopDevice();           // Stops all reception
    incrementStopCounter();
    return;  // All packets dropped until restart
}

This logic is CORRECT - when no receive buffers available, stop processing.

Data Available Check (Line 51083)

IF A NBIT 0 OR A/\60000><0 THEN % IF bit 0 clear OR (A AND 0x6000) != 0

Analysis:

if ((rrts & 0x0001) == 0 ||     // No data available
    (rrts & 0x6000) != 0)       // X.21 error (bits 13-14)
{
    return;  // Drop packet
}

This logic is CORRECT - only process when data available and no X.21 errors.

ROOT CAUSE IDENTIFIED

The Real Bug: HX21S Constant Wrong Value

Expected X.21 Logic:

// Should be:
const ushort HX21S_CORRECT = 0x4000;  // Bit 14 - X.21 Clear Indication

if ((rrts & 0x6000) != 0)  // X.21 error detected
{
    if ((rrts & 0x4000) != 0)  // Actual X.21 clear indication
    {
        terminateConnection();  // Only when DCE sends clear
    }
}

Actual (Buggy) Logic:

// Currently:
const ushort HX21S_WRONG = 0x000E;   // Bits 1,2,3 - Normal operation flags!

if ((rrts & 0x6000) != 0)  // X.21 error detected  
{
    if ((rrts & 0x000E) != 0)  // Tests DataAvailable, StatusAvailable, ReceiverActive
    {
        terminateConnection();  // Terminates on NORMAL data!
    }
}

Impact on Your Issues

Reception Problems:

1. False X.21 Clear Triggering: When data arrives (bit 0 set), HX21S test (0x000E) triggers connection termination
2. Packets Dropped: Connection terminated inappropriately, causing packet loss

Transmission Problems:

• Transmission logic is actually CORRECT - the SILFO+TXUND check properly detects errors
• Retransmission issue might be elsewhere - possibly in timing or multiple status reads

Recommended Fixes

1. Fix HX21S Constant:

   HX21S=004000  % Should be bit 14, not bits 1-3
   

2. Verify X.21 Clear Logic:

   if ((rrts & 0x6000) != 0)  // X.21 error
   {
       if ((rrts & 0x4000) != 0)  // Real X.21 clear (bit 14)
       {
           terminateConnection();
       }
   }
   

3. Check Status Read Timing:

   • Ensure status isn't read multiple times (clearing DMA bits)
   • Verify interrupt timing vs status register access

The HX21S constant being 0x000E instead of 0x4000 is likely the primary cause of your packet reception issues!