HDLC Protocol Implementation¶

LAPB, X.25, and protocol-level analysis for SINTRAN HDLC

Protocol Layers¶

LAPB (Link Access Procedure Balanced)¶

Layer: Data Link Layer (Layer 2)
Standard: ISO/IEC 13239
Mode: Balanced (both stations equal)

LAPB Frame Structure:

FLAG | ADDRESS | CONTROL | DATA | FCS | FLAG
0x7E | 1 byte  | 1 byte  | N    | 2   | 0x7E

Control Field Types: - I-frames (Information): Bit 0 = 0, carry user data + sequence numbers - S-frames (Supervisory): Bits 0-1 = 01, flow control (RR, RNR, REJ) - U-frames (Unnumbered): Bits 0-1 = 11, link management (SABM, DISC, UA, DM)

X.25 Protocol¶

Layer: Network Layer (Layer 3)
Runs over: LAPB (Layer 2)
Requires: X.21 interface for call setup

X.25 Packet Types: - Call Setup: CALL REQUEST, CALL ACCEPT - Data Transfer: DATA packets - Flow Control: RR (Receive Ready), RNR (Receive Not Ready) - Call Clearing: CLEAR REQUEST, CLEAR CONFIRMATION

Protocol Mode Detection¶

SINTRAN supports both modes:

Pure LAPB Mode (Current implementation): - No X.21 signaling required - Simple frame exchange - Bits 13-14 (X21D/X21S) kept clear - HX21S bits (1-3) not used

X.25 Network Mode (Advanced): - Full X.21 interface required - Call setup/clearing via X.21 - HX21S bits (1-3) track receiver state - Connection management needed

SINTRAN Protocol Variables¶

Key Variables¶

Variable	Purpose	Values
INTSTA	Interface Status	0=uninitialized, 2=initialized
ACTSW	Active Switch	0=idle, 1=active
HASTAT	Hardware Status	Last RRTS/RTTS value
XRETRY	Retry Counter	0-MAXRETRY
LISTP	List Pointer	Current DMA descriptor
LIINT	List Interrupt	DMA list interrupt pointer

State Tracking¶

% Initialization check
IF INTSTA >< 2 THEN
   A:=ENINIT; GO BACKX  % Error: not initialized
FI

% Activity check
IF ACTSW = 0 THEN
   % Device idle - can start operation
ELSE
   % Device busy - queue or wait
FI

Pseudocode Reference¶

Transmission Pseudocode (XSSDATA)¶

XSSDATA:
    1. Check INTSTA == 2 (initialized)
    2. Set LISTP = LIINT (DMA list pointer)
    3. Build DMA descriptor with LKEY = FSERM (0x1003)
    4. Calculate physical address of descriptor
    5. Write address to WDMA (IOX+15)
    6. Write DMA command 0x0400 to WDCR (IOX+17)
    7. Enable transmitter via WTTC (IOX+13) = 0x025C
    8. Set ACTSW = 1 (mark active)
    9. Wait for HOINT interrupt
    10. Check RTTS: success if (RTTS & 0x8002) == 0
    11. If error: retry up to MAXRETRY times
    12. Return status to caller

Reception Pseudocode (XSSREC)¶

XSSREC:
    1. Check INTSTA == 2 (initialized)
    2. Allocate receive buffer list
    3. Set LKEY = 0x0400 (empty receiver block)
    4. Write buffer address to WDMA
    5. Write DMA command 0x0201 to WDCR
    6. Enable receiver via WRTC (IOX+11)
    7. Set ACTSW = 1
    8. Wait for HIINT interrupt
    9. Check RRTS:
       - If bit 0 = 0: no data, ignore
       - If bits 13-14 != 0: X.21 error, handle
       - If bit 11 = 1: buffer empty, stop
    10. Process received data from buffers
    11. Return data to caller

PAD Connection Protocol¶

PAD: Packet Assembler/Disassembler - converts between async terminal and packet-switched network

PAD Connection Sequence¶

1. Terminal connects to PAD (async serial)
2. PAD establishes X.25 call to remote host
3. PAD assembles terminal characters into X.25 packets
4. PAD disassembles X.25 packets to terminal characters
5. On disconnect: PAD sends CLEAR REQUEST

SINTRAN PAD Support¶

SINTRAN handles PAD connections through: - LAPB layer: Reliable frame transport - X.25 layer: Packet encapsulation - TAD protocol: Terminal access device extensions

Protocol Constants¶

Frame Constants¶

Constant	Value	Purpose
FLAG	0x7E	Frame delimiter
FSERM	0x1003	Single frame LKEY (TSOM+TEOM)
MAXRETRY	3-5	Maximum transmission retries

Error Codes¶

Code	Octal	Meaning
ENINIT	236	Device not initialized
ETRANS	237	Transmission failure
EUND	102	Underrun error (bits 1,6)

Protocol State Machines¶

Transmission State Machine¶

IDLE
  ↓ [User sends data]
SETUP_DMA
  ↓ [Write registers]
TRANSMITTING (ACTSW=1)
  ↓ [Level 12 interrupt]
CHECK_STATUS
  ├─ [(RTTS & 0x8002) == 0] → SUCCESS → IDLE
  └─ [(RTTS & 0x8002) != 0] → ERROR → RETRY or FAIL

Reception State Machine¶

IDLE
  ↓ [Setup buffers]
RECEIVING (ACTSW=1)
  ↓ [Level 13 interrupt]
CHECK_STATUS
  ├─ [RRTS bit 0 = 1, no errors] → PROCESS_DATA → RECEIVING
  ├─ [RRTS bit 11 = 1] → BUFFER_EMPTY → IDLE
  └─ [RRTS bits 13-14 != 0] → X21_ERROR → HANDLE_ERROR

See Also: - 01-HDLC-Hardware-Reference.md - X.21 protocol details - 04-HDLC-Interrupt-Handlers.md - State machine implementation - 06-HDLC-Emulator-Guide.md - Complete C# implementation

Document Version: 2.0 (Consolidated)
Source Files: SINTRAN_HDLC_Complete_Pseudocode.md, SINTRAN_HDLC_Pseudocode.md, LAPB_vs_X25_Protocol_Handling.md, PAD_Connection_Deep_Analysis.md, HDLC_Variable_Reference.md, SINTRAN_Variable_Name_Analysis.md