diff --git a/chip-z80/support.js b/chip-z80/support.js
index f8859dd..14c9869 100644
--- a/chip-z80/support.js
+++ b/chip-z80/support.js
@@ -110,22 +110,20 @@ var state        = 0;
 var last_rd_done = 1;
 
 function handleBusRead(){
-    if(!isNodeHigh(nodenames['_m1']) && !isNodeHigh(nodenames['_iorq'])) {
-        // Interrupt acknownledge cycle, force 0xFF onto the bus
-        // In IM0 this is seen as RST 0x38
-        // In IM1 this is ignored
-        // In IM2 this is used as the low byte of the vector
-        // TODO: ideally this "vector" would be a configurable parameter
-        writeDataBus(0xff);
-    } else if(!isNodeHigh(nodenames['_rd'])){
+    if(!isNodeHigh(nodenames['_rd']) && !isNodeHigh(nodenames['_mreq'])) {
+        // Memory read
         var a = readAddressBus();
         var d = eval(readTriggers[a]);
         if(d == undefined)
             d = mRead(readAddressBus());
-        if(!isNodeHigh(nodenames['_m1']) && !isNodeHigh(nodenames['_mreq'])) {
+        if(!isNodeHigh(nodenames['_m1'])) {
             eval(fetchTriggers[d]);
         }
         writeDataBus(d);
+    } else {
+        // In all other cases we set the data bus to FF
+        // as a crude indicateion that it's not being driven
+        writeDataBus(0xff);
     }
 
     // Prefix / displacement / opcode state machine, deals with: