diff --git a/PDP11/pdp11_dmc.c b/PDP11/pdp11_dmc.c
index 2f91d6ec..56f6a006 100644
--- a/PDP11/pdp11_dmc.c
+++ b/PDP11/pdp11_dmc.c
@@ -2309,12 +2309,18 @@ while ((control = controller->control_out)) {
 controller->control_out = NULL;
 dmc_setreg(controller, 0, 0, DBG_RGC);
 if (controller->dev_type == DMR) {
-    /* Indicates microdiagnostics complete */
-    if (((*controller->csrs->sel0 & DMC_SEL0_M_UDIAG) != 0) ^
-        (dmc_microdiag[controller->index]))
-        dmc_setreg(controller, 2, 0x8000, DBG_RGC);/* Microdiagnostics Complete */
-    else
-        dmc_setreg(controller, 2, 0x4000, DBG_RGC); /* Microdiagnostics Inhibited */
+    if (dmc_is_attached(controller->unit)) {
+        /* Indicates microdiagnostics complete */
+        if (((*controller->csrs->sel0 & DMC_SEL0_M_UDIAG) != 0) ^
+            (dmc_microdiag[controller->index]))
+            dmc_setreg(controller, 2, 0x8000, DBG_RGC);/* Microdiagnostics Complete */
+        else
+            dmc_setreg(controller, 2, 0x4000, DBG_RGC); /* Microdiagnostics Inhibited */
+        }
+    else {
+        /* Indicate M8203 (Line Unit) test failed */
+        dmc_setreg(controller, 2, 0x0200, DBG_RGC);
+        }
     }
 else {
     /* preserve contents of BSEL3 if DMC-11 */