diff --git a/cpu/avr/dev/rs232.c b/cpu/avr/dev/rs232.c
index ce4fed3c7..2df263491 100644
--- a/cpu/avr/dev/rs232.c
+++ b/cpu/avr/dev/rs232.c
@@ -85,7 +85,7 @@
 
 #if defined (__AVR_ATmega128__) || defined(__AVR_ATmega1284P__) || defined(__AVR_ATmega1281__) || defined(__AVR_ATmega128RFA1__) || defined(__AVR_ATmega128RFR2__) || defined(__AVR_ATmega256RFR2__)
 #ifndef NUMPORTS
-#define NUMPORTS 1 // we use the second serial for arduino api HardwareSerial1.cpp
+#define NUMPORTS 2
 #elif NUMPORTS > 2
 #error Only two serial ports are defined for this processor!
 #endif
diff --git a/platform/osd-merkur-128/contiki-conf.h b/platform/osd-merkur-128/contiki-conf.h
index 0b2d8ebb1..868c0cd4b 100644
--- a/platform/osd-merkur-128/contiki-conf.h
+++ b/platform/osd-merkur-128/contiki-conf.h
@@ -66,6 +66,8 @@ typedef uint32_t clock_time_t;
 void clock_delay_msec(uint16_t howlong);
 void clock_adjust_ticks(clock_time_t howmany);
 
+/* We use the second serieal with the arduino api */
+#define NUMPORTS 1
 /* COM port to be used for SLIP connection. This is usually UART0. */
 #define SLIP_PORT RS232_PORT_0
 
diff --git a/platform/osd-merkur-256/contiki-conf.h b/platform/osd-merkur-256/contiki-conf.h
index 6c1965274..db09f3cac 100644
--- a/platform/osd-merkur-256/contiki-conf.h
+++ b/platform/osd-merkur-256/contiki-conf.h
@@ -67,6 +67,8 @@ typedef uint32_t clock_time_t;
 void clock_delay_msec(uint16_t howlong);
 void clock_adjust_ticks(clock_time_t howmany);
 
+/* We use the second serieal with the arduino api */
+#define NUMPORTS 1
 /* COM port to be used for SLIP connection. This is usually UART0. */
 #define SLIP_PORT RS232_PORT_0