Category : Modula II Source Code
Archive   : PCKERM.ZIP
Filename : DATALINK.MOD

IMPLEMENTATION MODULE DataLink; (* Sends and Receives Packets for PCKermit *)

FROM InOut IMPORT
WriteString, WriteLn;

FROM Delay IMPORT
Delay; (* delay is in milliseconds *)

FROM BitByteOps IMPORT
ByteAnd;

IMPORT RS232Int; (* for RS232Int.BusyRead, RS232Int.Write *)

FROM PAD IMPORT
PacketType, yourNPAD, yourPADC, yourEOL;

IMPORT ASCII;


CONST
MAXtime = 10000;
MAXsohtrys = 100;

VAR
ch : CHAR;
GotChar : BOOLEAN;


PROCEDURE Char (c : INTEGER) : CHAR;
(* converts a number 0-95 into a printable character *)
BEGIN
RETURN (CHR (CARDINAL (ABS (c) + 32)));
END Char;


PROCEDURE UnChar (c : CHAR) : INTEGER;
(* converts a character into its corresponding number *)
BEGIN
RETURN (ABS (INTEGER (ORD (c)) - 32));
END UnChar;


PROCEDURE FlushUART;
(* ensure no characters left in UART holding registers *)
BEGIN
Delay (500);
REPEAT
RS232Int.BusyRead (ch, GotChar);
UNTIL NOT GotChar;
END FlushUART;


PROCEDURE SendPacket (s : PacketType);
(* Adds SOH and CheckSum to packet *)

VAR
i : INTEGER;
checksum : INTEGER;

BEGIN
Delay (10); (* give host a chance to catch its breath *)
FOR i := 1 TO yourNPAD DO
RS232Int.Write (yourPADC);
END;
RS232Int.Write (ASCII.soh);
i := 1;
checksum := 0;
WHILE s[i] # 0C DO
INC (checksum, ORD (s[i]));
RS232Int.Write (s[i]);
INC (i);
END;
checksum := checksum + (INTEGER (BITSET (checksum) * {7, 6}) DIV 64);
checksum := INTEGER (BITSET (checksum) * {5, 4, 3, 2, 1, 0});
RS232Int.Write (Char (checksum));
IF yourEOL # 0C THEN
RS232Int.Write (yourEOL);
END;
END SendPacket;


PROCEDURE ReceivePacket (VAR r : PacketType) : BOOLEAN;
(* strips SOH and checksum -- return FALSE if timed out or bad checksum *)

VAR
sohtrys, time : INTEGER;
i, len : INTEGER;
ch : CHAR;
checksum : INTEGER;
mycheck, yourcheck : CHAR;

BEGIN
sohtrys := MAXsohtrys;
REPEAT
time := MAXtime;
REPEAT
DEC (time);
RS232Int.BusyRead (ch, GotChar);
UNTIL GotChar OR (time = 0);
ch := CHAR (ByteAnd (ch, 177C)); (* mask off MSB *)
(* skip over up to MAXsohtrys padding characters, *)
(* but allow only MAXsohtrys/10 timeouts *)
IF GotChar THEN
DEC (sohtrys);
ELSE
DEC (sohtrys, 10);
END;
UNTIL (ch = ASCII.soh) OR (sohtrys <= 0);

IF ch = ASCII.soh THEN
(* receive rest of packet *)
time := MAXtime;
REPEAT
DEC (time);
RS232Int.BusyRead (ch, GotChar);
UNTIL GotChar OR (time = 0);
ch := CHAR (ByteAnd (ch, 177C));
len := UnChar (ch);
r[1] := ch;
checksum := ORD (ch);
i := 2; (* on to second character in packet -- after LEN *)
REPEAT
time := MAXtime;
REPEAT
DEC (time);
RS232Int.BusyRead (ch, GotChar);
UNTIL GotChar OR (time = 0);
ch := CHAR (ByteAnd (ch, 177C));
r[i] := ch; INC (i);
INC (checksum, (ORD (ch)));
UNTIL (i > len);
time := MAXtime;
REPEAT
DEC (time);
RS232Int.BusyRead (ch, GotChar);
UNTIL GotChar OR (time = 0); (* get checksum character *)
ch := CHAR (ByteAnd (ch, 177C));
yourcheck := ch;
r[i] := 0C;
checksum := checksum +
(INTEGER (BITSET (checksum) * {7, 6}) DIV 64);
checksum := INTEGER (BITSET (checksum) * {5, 4, 3, 2, 1, 0});
mycheck := Char (checksum);
IF mycheck = yourcheck THEN (* checksum OK *)
RETURN TRUE;
ELSE (* ERROR!!! *)
WriteString ("Bad Checksum"); WriteLn;
RETURN FALSE;
END;
ELSE
WriteString ("No SOH"); WriteLn;
RETURN FALSE;
END;
END ReceivePacket;

END DataLink.