diff --git a/dda_queue.c b/dda_queue.c
index 84a68e2..e5a0cc5 100644
--- a/dda_queue.c
+++ b/dda_queue.c
@@ -3,6 +3,7 @@
 #include	<string.h>
 #include	<avr/interrupt.h>
 
+#include	"machine.h" // for XONXOFF
 #include	"timer.h"
 #include	"serial.h"
 #include	"sermsg.h"
@@ -70,9 +71,9 @@ void enqueue(TARGET *t) {
 	mb_head = h;
 
 	#ifdef	XONXOFF
-		// if queue is full, stop transmition
-		if (queue_full())
-			xoff();
+	// if queue is full, stop transmission
+	if (queue_full())
+		xoff();
 	#endif
 
 	// fire up in case we're not running yet
@@ -104,9 +105,9 @@ void enqueue_temp_wait() {
 	mb_head = h;
 
 	#ifdef	XONXOFF
-		// if queue is full, stop transmition
-		if (queue_full())
-			xoff();
+	// if queue is full, stop transmission
+	if (queue_full())
+		xoff();
 	#endif
 
 	// fire up in case we're not running yet
@@ -126,8 +127,8 @@ void next_move() {
 	}
 
 	#ifdef	XONXOFF
-		// restart transmission
-		xon();
+	// restart transmission
+	xon();
 	#endif
 }
 
diff --git a/gcode.c b/gcode.c
index ac03a53..a4770a3 100644
--- a/gcode.c
+++ b/gcode.c
@@ -414,7 +414,7 @@ void process_gcode_command(GCODE_COMMAND *gcmd) {
 			//	G4 - Dwell
 			case 4:
 				#ifdef	XONXOFF
-					xoff();
+				xoff();
 				#endif
 				// wait for all moves to complete
 				for (;queue_empty() == 0;)
@@ -422,7 +422,7 @@ void process_gcode_command(GCODE_COMMAND *gcmd) {
 				// delay
 				delay_ms(gcmd->P);
 				#ifdef	XONXOFF
-					xon();
+				xon();
 				#endif
 				break;
 
diff --git a/machine.h b/machine.h
index 13ff441..033ec7c 100644
--- a/machine.h
+++ b/machine.h
@@ -60,7 +60,9 @@
 // this should help immensely with dropped serial characters, but may also make debugging infuriating due to the complexities arising from nested interrupts
 #define		STEP_INTERRUPT_INTERRUPTIBLE	1
 
-// Xon/Xoff flow control. Should be redundant
+// Xon/Xoff flow control. Redundant when using RepRap Host for sending GCode,
+// but mandatory when sending GCode files with a plain terminal emulator,
+// like GtkTerm (Linux), CoolTerm (Mac) or HyperTerminal (Windows).
 // #define	XONXOFF
 
 /*
diff --git a/serial.c b/serial.c
index 63d66e3..d094ee1 100644
--- a/serial.c
+++ b/serial.c
@@ -45,11 +45,14 @@ volatile uint8_t txbuf[BUFSIZE];
 	data = buf[tail++]; tail &= (BUFSIZE - 1);
 */
 
-volatile uint8_t flowflags = 0;
-#define		FLOWFLAG_SEND_XOFF	1
-#define		FLOWFLAG_SEND_XON		2
-#define		FLOWFLAG_SENT_XOFF	4
-#define		FLOWFLAG_SENT_XON		8
+#ifdef	XONXOFF
+#define		FLOWFLAG_SEND_XON		1
+#define		FLOWFLAG_SEND_XOFF	2
+#define		FLOWFLAG_SENT_XON		4
+#define		FLOWFLAG_SENT_XOFF	8
+// initially, send an XON
+volatile uint8_t flowflags = FLOWFLAG_SEND_XON;
+#endif
 
 void serial_init()
 {
@@ -82,15 +85,15 @@ ISR(USART_RX_vect)
 
 ISR(USART_UDRE_vect)
 {
-	#if XONXOFF
-	if (flowflags & FLOWFLAG_SEND_XOFF) {
-		UDR0 = ASCII_XOFF;
-		flowflags = (flowflags & ~FLOWFLAG_SEND_XOFF) | FLOWFLAG_SENT_XOFF;
-	}
-	else if (flowflags & FLOWFLAG_SEND_XON) {
+	#ifdef	XONXOFF
+	if (flowflags & FLOWFLAG_SEND_XON) {
 		UDR0 = ASCII_XON;
 		flowflags = (flowflags & ~FLOWFLAG_SEND_XON) | FLOWFLAG_SENT_XON;
 	}
+	else if (flowflags & FLOWFLAG_SEND_XOFF) {
+		UDR0 = ASCII_XOFF;
+		flowflags = (flowflags & ~FLOWFLAG_SEND_XOFF) | FLOWFLAG_SENT_XOFF;
+	}
 	else
 	#endif
 	if (buf_canread(tx))
@@ -189,16 +192,16 @@ void serial_writestr_P(PGM_P data)
 }
 
 #ifdef	XONXOFF
-	void xon() {
-		if (flowflags & FLOWFLAG_SENT_XOFF)
-			flowflags = FLOWFLAG_SEND_XON;
-		// enable TX interrupt so we can send this character
-		UCSR0B |= MASK(UDRIE0);
-	}
+void xon() {
+	if (flowflags & FLOWFLAG_SENT_XOFF)
+		flowflags = FLOWFLAG_SEND_XON;
+	// enable TX interrupt so we can send this character
+	UCSR0B |= MASK(UDRIE0);
+}
 
-	void xoff() {
-		flowflags = FLOWFLAG_SEND_XOFF;
-		// enable TX interrupt so we can send this character
-		UCSR0B |= MASK(UDRIE0);
-	}
+void xoff() {
+	flowflags = FLOWFLAG_SEND_XOFF;
+	// enable TX interrupt so we can send this character
+	UCSR0B |= MASK(UDRIE0);
+}
 #endif
diff --git a/serial.h b/serial.h
index 73a7b18..6deccf3 100644
--- a/serial.h
+++ b/serial.h
@@ -5,6 +5,8 @@
 #include	<avr/io.h>
 #include	<avr/pgmspace.h>
 
+#include	"machine.h" // for XONXOFF
+
 // initialise serial subsystem
 void serial_init(void);
 
@@ -28,9 +30,9 @@ void serial_writeblock_P(PGM_P data, int datalen);
 void serial_writestr_P(PGM_P data);
 
 #ifdef	XONXOFF
-	// XON/XOFF flow control
-	void xoff(void);
-	void xon(void);
+// XON/XOFF flow control
+void xon(void);
+void xoff(void);
 #endif
 
 #endif	/* _SERIAL_H */