diff --git a/sys/i386/isa/tw.c b/sys/i386/isa/tw.c
index 64671ccc3a24..603840bd1464 100644
--- a/sys/i386/isa/tw.c
+++ b/sys/i386/isa/tw.c
@@ -931,7 +931,7 @@ int unit;
    * Ignore any interrupts that occur if the device is not open.
    */
   if(sc->sc_state == 0) return;
-  newphase = inb(port + tw_control) & TWC_SYNC;
+  newphase = inb(port + tw_zcport) & tw_zcmask;
   microtime(&tv);
 
   /*