diff --git a/system/config.v b/system/config.v
index a475d49..02a6404 100644
--- a/system/config.v
+++ b/system/config.v
@@ -47,6 +47,11 @@
 `define L1_CACHE_SIZE 4
 
 
+// Use high impedance instead of some other undefined value
+// when a signal is invalid (for example a data port when a
+// memory is not selected).  This could be used for debug
+//`define USE_HIGH_IMPEDANCE
+
 /********** Internal **********/
 `ifdef SYNTHESIS
 	`undef DEBUG_REG_WRITES
diff --git a/system/memory.v b/system/memory.v
index b655a80..cc79d1b 100644
--- a/system/memory.v
+++ b/system/memory.v
@@ -53,10 +53,13 @@ initial begin
 end
 
 
-
-assign cpu_read_data[7:0] = !address[0:0] & !rd & !cs ? memory[address[ADDRESS_WIDTH-1:1]][15:8]  : 8'hz;
-
-assign cpu_read_data[15:8] =     !BHE     & !rd & !cs ? memory[address[ADDRESS_WIDTH-1:1]][ 7:0]  : 8'hz;
+`ifdef USE_HIGH_IMPEDANCE
+assign cpu_read_data[7:0]  = !address[0:0] & !rd & !cs ? memory[address[ADDRESS_WIDTH-1:1]][15:8]  : `16'hz;
+assign cpu_read_data[15:8] =     !BHE      & !rd & !cs ? memory[address[ADDRESS_WIDTH-1:1]][ 7:0]  : `16'hz;
+`else
+assign cpu_read_data[7:0]  = memory[address[ADDRESS_WIDTH-1:1]][15:8];
+assign cpu_read_data[15:8] = memory[address[ADDRESS_WIDTH-1:1]][ 7:0];
+`endif
 
 
 always @(posedge clock) begin