the ibrt long message protocol

the fields

• [SYN++] is at least 2 SYN character 0x16
• STX is start of message (1 byte) = 0x02
• Len is 2 bytes counting the number of bytes not including the SYN, low byte first
• SRC is the source ID (1 byte)
• DST is the destination ID (1 byte)
• Cmd is a predefined command code (1 byte)
• [Data] is optional of any length depending on the command code
• CRC is a 16 bit CRC, low byte first

• a slave device only answers to a command
• a device only replies if the ID matches and if the CRC is correct
• a reply has the same command code as the issued command, but the data field can be composed differently.
• a reply has the SRC and DST fields swapped.
• the broadcast ID to which all devices answer is 0xFF
• the command processing is stateless
• commands are quick to process

the CRC

PROCEDURE calcCRCbyte(data:BYTE;VAR crc:WORD);
VAR i:BYTE;
BEGIN
 FOR i:=0 TO 7 DO BEGIN
   IF ((data and $01)XOR(crc AND $0001)<>0) THEN BEGIN
     crc:=crc shr 1;
     crc:= crc XOR $A001;
    END
   ELSE BEGIN
     crc:=crc shr 1;
    END;
   data:=data shr 1;
  END;
END;

The command to be sent is assumed zero aligned, eg STX is on address 0, 
and SYN is not included in the CRC

FUNCTION CRC(VAR DATA;size:word):WORD;
VAR i:word;
BEGIN
 CRC:=0;
 FOR i:=0 TO size-1 DO BEGIN
   calcCRCbyte(DATA[i],CRC);
 END;
END;

the receiving state machine

• 0 : idle - wait for SYN
• 1 : wait for STX
• 2 : read LenLo
• 3 : read LenHi
• 4 : count down
• 0x0E : Buffer full
• 0x0F : Message received

interrupt rx_interrupt;
var u:byte;
begin
 u:=ReadUART;
 if (RxPtr=RxBufLen) then RxState:=0x0E;
 else begin
  case RxState of
   0:if u=SYN then RxState :=1; |
   1:case u of
      SYN:RxState :=1; |
      STX:RxBuf[0]:=STX; RxState :=2; |
      else state:=0;
     end; |
   2:RxBuf[1]:=u; LenLo:=u; RxState :=3; |
   3:RxBuf[2]:=u; LenHi:=u; Rxcount:=2; RxState :=4; |
   4:If RxCount<>RxLen then begin
      RxBuf[RxCount]:=u;
      if RxCount < BufLen then inc(RxCount);
      end;
     else RxState :=0x0F; |
  end; // case
 end; // buf not full
end; // interrupt

procedure ComLoop;
begin
 if RxState=0x0E then begin  // discard content
  RxState:=0;
  end;
 if RxState=0x0F then begin
  RxCRCMessage(rxptr-2); // CRC is updated
  if (CRChigh=RxBuf[rxptr-2])and
     (CRCLow=RxBuf[rxptr-1]) then begin// crc ok.
   if (RxBuf[4]=DeviceID)or(RxBuf[4]=$FF) then begin
    src:=RxBuf[3];
    dst:=RxBuf[4];
    cmd:=RxBuf[5];
     case cmd of
      0x00:// Echo
         CopyRxTx;
         AddCrc;
         SetupTx;|
      ..
      0x44://UserString read
         txlen:=ord(UserString[0])+10;
         SetupTxFrame;
         CopyBlock(@UserString,@TxBuf+7,ord(UserString[0])+1);
         AddCrc;
         SetupTx;|
     end; //case
    end; // ID ok
   end; // crc ok
  RxState:=0;
  RxPtr:=0;
  end; // 0x0F
end;

independent implementation details

• baudrate 115200 or faster
• other serial parameters 8,n,1
• buffer length 560 or 1080 bytes
• the underlying hardware is RS232 or RS422

predefined commands

further