diff --git a/VAX/vax610_stddev.c b/VAX/vax610_stddev.c
index 153da770..698d80b9 100644
--- a/VAX/vax610_stddev.c
+++ b/VAX/vax610_stddev.c
@@ -74,6 +74,7 @@ extern jmp_buf save_env;
 extern int32 p1;
 
 int32 tti_csr = 0;                                      /* control/status */
+uint32 tti_buftime;                                     /* time input character arrived */
 int32 tto_csr = 0;                                      /* control/status */
 int32 tto_leds = 0;                                     /* processor board LEDs */
 int32 clk_csr = 0;                                      /* control/status */
@@ -332,7 +333,11 @@ t_stat tti_svc (UNIT *uptr)
 {
 int32 c;
 
-sim_activate (uptr, KBD_WAIT (uptr->wait, tmr_poll));   /* continue poll */
+sim_clock_coschedule (uptr, KBD_WAIT (uptr->wait, tmr_poll));
+                                                        /* continue poll */
+if ((tti_csr & CSR_DONE) &&                             /* input still pending and < 500ms? */
+    ((sim_os_msec () - tti_buftime) < 500))
+     return SCPE_OK;
 if ((c = sim_poll_kbd ()) < SCPE_KFLAG)                 /* no char or error? */
     return c;
 if (c & SCPE_BREAK) {                                   /* break? */
@@ -341,6 +346,7 @@ if (c & SCPE_BREAK) {                                   /* break? */
     tti_unit.buf = TTIBUF_ERR | TTIBUF_FRM | TTIBUF_RBR;
     }
 else tti_unit.buf = sim_tt_inpcvt (c, TT_GET_MODE (uptr->flags));
+tti_buftime = sim_os_msec ();
 uptr->pos = uptr->pos + 1;
 tti_csr = tti_csr | CSR_DONE;
 if (tti_csr & CSR_IE)
diff --git a/VAX/vax630_stddev.c b/VAX/vax630_stddev.c
index 95751080..6db71b1f 100644
--- a/VAX/vax630_stddev.c
+++ b/VAX/vax630_stddev.c
@@ -52,6 +52,7 @@ extern int32 int_req[IPL_HLVL];
 extern int32 hlt_pin;
 
 int32 tti_csr = 0;                                      /* control/status */
+uint32 tti_buftime;                                     /* time input character arrived */
 int32 tto_csr = 0;                                      /* control/status */
 int32 clk_csr = 0;                                      /* control/status */
 int32 clk_tps = 100;                                    /* ticks/second */
@@ -267,7 +268,11 @@ t_stat tti_svc (UNIT *uptr)
 {
 int32 c;
 
-sim_activate (uptr, KBD_WAIT (uptr->wait, tmr_poll));   /* continue poll */
+sim_clock_coschedule (uptr, KBD_WAIT (uptr->wait, tmr_poll));
+                                                        /* continue poll */
+if ((tti_csr & CSR_DONE) &&                             /* input still pending and < 500ms? */
+    ((sim_os_msec () - tti_buftime) < 500))
+     return SCPE_OK;
 if ((c = sim_poll_kbd ()) < SCPE_KFLAG)                 /* no char or error? */
     return c;
 if (c & SCPE_BREAK) {                                   /* break? */
@@ -276,6 +281,7 @@ if (c & SCPE_BREAK) {                                   /* break? */
     tti_unit.buf = TTIBUF_ERR | TTIBUF_FRM | TTIBUF_RBR;
     }
 else tti_unit.buf = sim_tt_inpcvt (c, TT_GET_MODE (uptr->flags));
+tti_buftime = sim_os_msec ();
 uptr->pos = uptr->pos + 1;
 tti_csr = tti_csr | CSR_DONE;
 if (tti_csr & CSR_IE)
diff --git a/VAX/vax730_stddev.c b/VAX/vax730_stddev.c
index 72287b94..0e060a70 100644
--- a/VAX/vax730_stddev.c
+++ b/VAX/vax730_stddev.c
@@ -158,6 +158,7 @@
 #define TD_INIT         9                               /* empty buffer */
 
 int32 tti_csr = 0;                                      /* control/status */
+uint32 tti_buftime;                                     /* time input character arrived */
 int32 tti_buf = 0;                                      /* buffer */
 int32 tti_int = 0;                                      /* interrupt */
 int32 tto_csr = 0;                                      /* control/status */
@@ -660,12 +661,17 @@ t_stat tti_svc (UNIT *uptr)
 {
 int32 c;
 
-sim_activate (uptr, KBD_WAIT (uptr->wait, tmr_poll));   /* continue poll */
+sim_clock_coschedule (uptr, KBD_WAIT (uptr->wait, tmr_poll));
+                                                        /* continue poll */
+if ((tti_csr & CSR_DONE) &&                             /* input still pending and < 500ms? */
+    ((sim_os_msec () - tti_buftime) < 500))
+     return SCPE_OK;
 if ((c = sim_poll_kbd ()) < SCPE_KFLAG)                 /* no char or error? */
     return c;
 if (c & SCPE_BREAK)                                     /* break? */
     tti_buf = RXDB_ERR;
 else tti_buf = sim_tt_inpcvt (c, TT_GET_MODE (uptr->flags));
+tti_buftime = sim_os_msec ();
 uptr->pos = uptr->pos + 1;
 tti_csr = tti_csr | CSR_DONE;
 if (tti_csr & CSR_IE)
diff --git a/VAX/vax750_stddev.c b/VAX/vax750_stddev.c
index 8f3f5abf..b818681d 100644
--- a/VAX/vax750_stddev.c
+++ b/VAX/vax750_stddev.c
@@ -157,6 +157,7 @@
 #define TD_INIT         9                               /* empty buffer */
 
 int32 tti_csr = 0;                                      /* control/status */
+uint32 tti_buftime;                                     /* time input character arrived */
 int32 tti_buf = 0;                                      /* buffer */
 int32 tti_int = 0;                                      /* interrupt */
 int32 tto_csr = 0;                                      /* control/status */
@@ -653,12 +654,17 @@ t_stat tti_svc (UNIT *uptr)
 {
 int32 c;
 
-sim_activate (uptr, KBD_WAIT (uptr->wait, tmr_poll));   /* continue poll */
+sim_clock_coschedule (uptr, KBD_WAIT (uptr->wait, tmr_poll));
+                                                        /* continue poll */
+if ((tti_csr & CSR_DONE) &&                             /* input still pending and < 500ms? */
+    ((sim_os_msec () - tti_buftime) < 500))
+     return SCPE_OK;
 if ((c = sim_poll_kbd ()) < SCPE_KFLAG)                 /* no char or error? */
     return c;
 if (c & SCPE_BREAK)                                     /* break? */
     tti_buf = RXDB_ERR | RXDB_FRM;
 else tti_buf = sim_tt_inpcvt (c, TT_GET_MODE (uptr->flags));
+tti_buftime = sim_os_msec ();
 uptr->pos = uptr->pos + 1;
 tti_csr = tti_csr | CSR_DONE;
 if (tti_csr & CSR_IE)
diff --git a/VAX/vax780_stddev.c b/VAX/vax780_stddev.c
index 415f1b5d..f2d07da5 100644
--- a/VAX/vax780_stddev.c
+++ b/VAX/vax780_stddev.c
@@ -182,6 +182,7 @@
 #define CALC_DA(t,s) (((t) * FL_NUMSC) + ((s) - 1)) * FL_NUMBY
 
 int32 tti_csr = 0;                                      /* control/status */
+uint32 tti_buftime;                                     /* time input character arrived */
 int32 tti_buf = 0;                                      /* buffer */
 int32 tti_int = 0;                                      /* interrupt */
 int32 tto_csr = 0;                                      /* control/status */
@@ -475,13 +476,15 @@ t_stat tti_svc (UNIT *uptr)
 int32 c;
 
 sim_clock_coschedule (uptr, tmxr_poll);                 /* continue poll */
-if (tti_csr & CSR_DONE)                                 /* is last input processed yet? */
-    return SCPE_OK;                                     /* wait */
+if ((tti_csr & CSR_DONE) &&                             /* input still pending and < 500ms? */
+    ((sim_os_msec () - tti_buftime) < 500))
+     return SCPE_OK;
 if ((c = sim_poll_kbd ()) < SCPE_KFLAG)                 /* no char or error? */
     return c;
 if (c & SCPE_BREAK)                                     /* break? */
     tti_buf = RXDB_ERR | RXDB_FRM;
 else tti_buf = sim_tt_inpcvt (c, TT_GET_MODE (uptr->flags));
+tti_buftime = sim_os_msec ();
 uptr->pos = uptr->pos + 1;
 tti_csr = tti_csr | CSR_DONE;
 if (tti_csr & CSR_IE)
diff --git a/VAX/vax_stddev.c b/VAX/vax_stddev.c
index fd2a3368..a116f149 100644
--- a/VAX/vax_stddev.c
+++ b/VAX/vax_stddev.c
@@ -97,6 +97,7 @@ extern int32 int_req[IPL_HLVL];
 extern int32 hlt_pin;
 
 int32 tti_csr = 0;                                      /* control/status */
+uint32 tti_buftime;                                     /* time input character arrived */
 int32 tto_csr = 0;                                      /* control/status */
 int32 clk_csr = 0;                                      /* control/status */
 int32 clk_tps = 100;                                    /* ticks/second */
@@ -331,6 +332,9 @@ int32 c;
 
 sim_clock_coschedule (uptr, KBD_WAIT (uptr->wait, tmr_poll));
                                                         /* continue poll */
+if ((tti_csr & CSR_DONE) &&                             /* input still pending and < 500ms? */
+    ((sim_os_msec () - tti_buftime) < 500))
+     return SCPE_OK;
 if ((c = sim_poll_kbd ()) < SCPE_KFLAG)                 /* no char or error? */
     return c;
 if (c & SCPE_BREAK) {                                   /* break? */
@@ -339,6 +343,7 @@ if (c & SCPE_BREAK) {                                   /* break? */
     tti_unit.buf = TTIBUF_ERR | TTIBUF_FRM | TTIBUF_RBR;
     }
 else tti_unit.buf = sim_tt_inpcvt (c, TT_GET_MODE (uptr->flags));
+tti_buftime = sim_os_msec ();
 uptr->pos = uptr->pos + 1;
 tti_csr = tti_csr | CSR_DONE;
 if (tti_csr & CSR_IE)