add rtu response length check

appl/cmd/testmodbus.b

@@ -486,8 +486,8 @@ getreply(p: ref Port): (byte, ref RMmsg, int, string)
 	p.rdlock.obtain();
 	n := len p.avail;
 	if(n >= 4) {
-		(addr, pdu, crc, err) = modbus->rtuunpack(p.avail);
-		if(err == nil) {
+		(addr, pdu, crc, n, err) = modbus->rtuunpack(p.avail);
+		if(err == nil && n > 0) {
 			(t, m) := RMmsg.unpack(pdu);
 			if(t > 0)
 				 r = m;
@@ -496,6 +496,9 @@ getreply(p: ref Port): (byte, ref RMmsg, int, string)
 				sys->fprint(stderr, "RX <- %s\n", hexdump(p.avail));
 			}
 			p.avail = p.avail[n:];
+		} else if(err == Modbus->ERR_RTUCRC) {
+			if(n > 0)
+				p.avail = p.avail[n:];
 		}
 	}
 	p.rdlock.release();

appl/lib/modbus.b

@@ -777,27 +777,42 @@ rtupack(addr: byte, pdu: array of byte): array of byte
 	return atu;
 }
 
-rtuunpack(data: array of byte): (byte, array of byte, int, string)
+rtuunpack(data: array of byte): (byte, array of byte, int, int, string)
 {
 	e : string;
 	addr : byte;
 	pdu : array of byte;
 	c : int;
 	n := len data;
-	if(n >= 4) {
+	if(n > 4) {
 		addr = data[0];
-		pdu = data[1:n-2];
-		c = g16(data, n-2);
-		if(c != rtucrc(addr, pdu))
-			e = RTUCRC_ERROR;
+		f := int data[1];
+		if(f > 16r80) {		# Modbus error
+			pdu = data[1:3];
+			c = g16(data, 4);
+			if(c != rtucrc(addr, pdu))
+				e = ERR_RTUCRC;
+			n = 5;
+		} else {
+			l := int data[2];
+			if(l+5 > n) {
+				e = ERR_RTUINCOMPLETE;
+				n = 0;
+			} else {
+				pdu = data[1:l+3];
+				c = g16(data, l+3);
+				if(c != rtucrc(addr, pdu))
+					e = ERR_RTUCRC;
+				n = l+5;
+			}
+		}
 	} else
-		e = "too short for an rtu packet";
-	return (addr, pdu, c, e);
+		e = ERR_RTUINCOMPLETE;
+	return (addr, pdu, c, n, e);
 }
 
 rtucrc(addr: byte, pdu: array of byte): int
 {
-#	crc->reset(crc_state);
 	crc_state.crc = 16rFFFF;
 
 	n := len pdu + BIT8SZ;
@@ -813,13 +828,13 @@ crc16(crcs : ref CRCstate, buf : array of byte, nb : int) : int
 	n := nb;
 	if (n > len buf)
 		n = len buf;
-	crc := crcs.crc;
+	c := crcs.crc;
 	tab := crcs.crctab;
 	for (i := 0; i < n; i++) {
-		crc = (crc >> 8) ^ tab[int(byte crc ^ buf[i])];
+		c = (c >> 8) ^ tab[int(byte c ^ buf[i])];
 	}
-	crcs.crc = crc;
-	return crc;
+	crcs.crc = c;
+	return c;
 }
 
 rtucrc_test(addr: byte, pdu: array of byte): int

appl/mkfile

@@ -1,8 +1,8 @@
 <../mkconfig
 
 DIRS=\
-	cmd\
 	lib\
+	cmd\
 
 <$ROOT/mkfiles/mksubdirs
 

module/modbus.m

@@ -20,7 +20,8 @@
 	ModeTCP,
 	Maxmode:	con iota;
 
-	RTUCRC_ERROR:	con "CRC error";
+	ERR_RTUCRC:	con "CRC error";
+	ERR_RTUINCOMPLETE:	con "RTU incomlete";
 
 	# Function code types
 	Treadcoils,				# 0x01
@@ -223,7 +224,7 @@
 	init:	fn();
 
 	rtupack:	fn(addr: byte, pdu: array of byte): array of byte;
-	rtuunpack:	fn(data: array of byte): (byte, array of byte, int, string);
+	rtuunpack:	fn(data: array of byte): (byte, array of byte, int, int, string);
 	rtucrc:		fn(addr: byte, pdu: array of byte): int;
 	rtucrc_test:		fn(addr: byte, pdu: array of byte): int;
 };