CRITICAL CORRECTIONS: HX21S Bit Testing and KEY Field Interpretation¶

BREAKTHROUGH DISCOVERIES¶

1. DMA LKEY Field = COM5025 Register Values¶

Revolutionary Understanding: The DMA descriptor LKEY field's low 8 bits contain actual COM5025 chip register values that get written directly to the hardware. This is what the HDLC manual calls "Dataflow Cost."

% DMA Descriptor LKEY Structure:
% Bits 15-8: Block control (Empty=010, Full=011, ToTransmit=100, etc.)
% Bits 7-0:  COM5025 register value (TSOM, TEOM, RSOM, REOM detection bits)

% Example - FSERM = 002003₈:
FSERM = 002003₈ = 0000 1000 0000 0011
        │        └─ 003: COM5025 TSOM(1) + TEOM(1) = complete frame
        └─ 010: Block to be transmitted

2. MASSIVE ERROR CORRECTED: HX21S Bit Testing Logic¶

The Critical Misunderstanding:

HX21S = 000016₈  % This is 14 DECIMAL (bit number), not a bitmask!

WRONG Interpretation (Previous):

IF A BIT HX21S THEN    % Incorrectly thought: "test bits 1,2,3"
% Actually means: "test bit 14"

CORRECT Interpretation:

HX21S = 000016₈ = 14 decimal = bit position 14
IF A BIT HX21S THEN    % Means: "IF bit 14 in register A is 1 THEN"

Impact on All Analysis Documents¶

Before (WRONG):¶

"HX21S tests receiver state bits 1,2,3"
"HX21S = 0x000E is a bitmask"
"Tests StatusAvailable, ReceiverActive, SyncFlagReceived"

After (CORRECT):¶

"HX21S tests X.21 Clear Indication bit 14"
"HX21S = 14 (decimal) is a bit position number"
"Tests X21S (Clear Indication) - exactly what the name suggests"

Corrected SINTRAN Logic Flows¶

Receiver Processing (HIINT):¶

% CORRECTED X.21 error handling:
IF A/\ HX21M >< 0 THEN           % X.21 error detected (bits 13-14)
   IF A BIT HX21S THEN           % IF bit 14 (X21S Clear) is set
      HASTAT BONE BLDON=:HASTAT  % Terminate connection cleanly
   FI
FI

C# Implementation (CORRECTED):¶

// OLD (WRONG):
if ((rrts & 0x000E) != 0)  // Wrong - tested bits 1,2,3

// NEW (CORRECT):
if ((rrts & 0x4000) != 0)  // Correct - tests bit 14 (X21S Clear)
{
    // X.21 Clear Indication detected - terminate connection
    setBlockDone();
}

Key Constants Corrected¶

Constant	Value	Previous (Wrong)	Correct Interpretation
HX21S	000016₈	Bitmask 0x000E (bits 1,2,3)	Bit position 14
BLDON	000010₈	Bitmask 0x0008 (bit 3)	Bit position 8
XBLDN	000010₈	Bitmask 0x0008 (bit 3)	Bit position 8

Pattern Recognition: SINTRAN constants ending in single digits are bit position numbers, not bitmasks!

Frame Boundary Detection (SOLVED)¶

How SINTRAN Controls FrameEnd vs BlockEnd:¶

// Read DMA descriptor LKEY field:
ushort lkeyValue = ReadDMADescriptor(dmaAddress);

// Extract COM5025 control bits (low 8 bits):
byte com5025Bits = (byte)(lkeyValue & 0xFF);
bool hasTSOM = (com5025Bits & 0x01) != 0;  // Transmit Start of Message
bool hasTEOM = (com5025Bits & 0x02) != 0;  // Transmit End of Message

// Now you know exactly what to set:
if (hasTSOM && hasTEOM) {
    // FSERM case: Complete frame in single block
    SetRTTSBit("FrameEnd + BlockEnd");
} else if (hasTEOM) {
    // Final block of multi-block frame
    SetRTTSBit("FrameEnd + BlockEnd");
} else {
    // Intermediate block
    SetRTTSBit("BlockEnd");  // Frame continues
}

Expected Multi-Block Frame Constants¶

Based on corrected understanding:

% Single block (FSERM): Block + TSOM + TEOM
SingleFrame = 002003₈    % 010 (block) + 003 (TSOM=1, TEOM=1)

% Multi-block frame sequence:
FirstBlock = 002001₈     % 010 (block) + 001 (TSOM=1 only)
MiddleBlock = 002000₈    % 010 (block) + 000 (no flags)
LastBlock = 002002₈      % 010 (block) + 002 (TEOM=1 only)

Root Cause Analysis Complete¶

Why Reception Was Failing:¶

Wrong HX21S interpretation caused normal packets to trigger X.21 Clear termination
DataAvailable + StatusAvailable + SyncFlagReceived (bits 0,1,3) are NORMAL reception flags
Testing these as "X.21 Clear" terminated valid connections

Why Transmission Issues Occurred:¶

Unknown frame boundaries - couldn't distinguish BlockEnd from FrameEnd
COM5025 integration not understood - LKEY contains the actual control bits
Emulator assumptions instead of reading SINTRAN's explicit instructions

Impact on Debugging¶

All previous analysis assuming HX21S = 0x000E is INVALID

The correct logic means: - X.21 Clear Indication is properly isolated to bit 14 - Normal reception bits (0,1,3) are not confused with protocol termination - Frame boundary control is explicit in DMA descriptors - HDLC emulator can read SINTRAN's exact intentions from LKEY field

This resolves the fundamental misunderstanding about both bit testing syntax and hardware integration!