diff --git a/sim_tmxr.c b/sim_tmxr.c
index 05d74865..23d5f0f4 100644
--- a/sim_tmxr.c
+++ b/sim_tmxr.c
@@ -3086,8 +3086,8 @@ while (*tptr) {
         serport = sim_open_serial (destination, NULL, &r);
         if (serport != INVALID_HANDLE) {
             sim_close_serial (serport);
-            if (strchr (destination, ';') && mp->modem_control && !(sim_switches & SIM_SW_REST))
-                return sim_messagef (SCPE_ARG, "Serial line parameters must be set within simulated OS: %s\n", 1 + strchr (destination, ';'));
+            if (strchr (destination, ';') && (mp->modem_control || mp->port_speed_control) && !(sim_switches & SIM_SW_REST))
+                return sim_messagef (SCPE_ARG, "%sSerial line parameters can only be set within simulated OS: %s\n", dev_name, 1 + strchr (destination, ';'));
             }
         else {
             char *eptr;