diff --git a/asminc/lynx.inc b/asminc/lynx.inc
index d65b7f8a9..0d34e1c7c 100644
--- a/asminc/lynx.inc
+++ b/asminc/lynx.inc
@@ -259,16 +259,26 @@ SND_INTERRUPT = TIMER7_INTERRUPT
 
 INTRST      = $FD80
 INTSET      = $FD81
+
 MAGRDY0     = $FD84
 MAGRDY1     = $FD85
 AUDIN       = $FD86
 SYSCTL1     = $FD87
 MIKEYHREV   = $FD88
 MIKEYSREV   = $FD89
-IODIR       = $FD8A
-IODAT       = $FD8B
 
-SERCTL      = $FD8C
+IODIR          = $FD8A
+IODAT          = $FD8B
+; IODIR and IODAT bit definitions
+AUDIN_BIT      = $10  ; Note that there is also the address AUDIN
+READ_ENABLE    = $10  ; Same bit for AUDIN_BIT
+RESTLESS       = $08
+NOEXP          = $04  ; If set, redeye is not connected
+CART_ADDR_DATA = $02
+CART_POWER_OFF = $02  ; Same bit for CART_ADDR_DATA
+EXTERNAL_POWER = $01
+
+SERCTL   = $FD8C
 ; SERCTL bit definitions for write operations
 TXINTEN  = $80
 RXINTEN  = $40
diff --git a/libsrc/lynx/ser/lynx-comlynx.s b/libsrc/lynx/ser/lynx-comlynx.s
index 486981184..7201264b7 100644
--- a/libsrc/lynx/ser/lynx-comlynx.s
+++ b/libsrc/lynx/ser/lynx-comlynx.s
@@ -200,7 +200,18 @@ checkhs:
         ldy     #SER_PARAMS::HANDSHAKE  ; Handshake
         lda     (ptr1),y
         cmp     #SER_HS_NONE
+        beq     redeye_ok
+        cmp     #SER_HS_SW              ; Software handshake will check for connected redeye
         bne     invparameter
+
+        lda     IODAT
+        and     #NOEXP                  ; Check if redeye bit flag is unset
+        beq     redeye_ok
+        lda     #SER_ERR_NO_DEVICE      ; ComLynx cable is not inserted
+        ldx     #0
+        rts
+
+redeye_ok:
         lda     SERDAT
         lda     contrl
         ora     #RXINTEN|RESETERR       ; Turn on interrupts for receive
@@ -209,6 +220,7 @@ checkhs:
         .assert SER_ERR_OK = 0, error
         tax
         rts
+
 invparameter:
         lda     #SER_ERR_INIT_FAILED
         ldx     #0 ; return value is char