diff --git a/H316/h316_stddev.c b/H316/h316_stddev.c
index ac005af1..ffe2876a 100644
--- a/H316/h316_stddev.c
+++ b/H316/h316_stddev.c
@@ -424,6 +424,8 @@ t_stat r;
 
 if (!(uptr->flags & UNIT_ATTABLE))                      /* not tti,tto */
     return SCPE_NOFNC;
+if (strcmp ("PTR", sim_uname (uptr)) == 0)              /* PTR is read only */
+    sim_switches |= SWMASK ('R');
 if ((r = attach_unit (uptr, cptr)))
     return r;
 if (sim_switches & SWMASK ('A'))                        /* -a? ASCII */
diff --git a/LGP/lgp_stddev.c b/LGP/lgp_stddev.c
index 278d3cf8..416fe183 100644
--- a/LGP/lgp_stddev.c
+++ b/LGP/lgp_stddev.c
@@ -618,6 +618,8 @@ t_stat tap_attach (UNIT *uptr, CONST char *cptr)
 {
 t_stat r;
 
+if (strcmp ("PTR", sim_uname (uptr)) == 0)
+    sim_switches |= SWMASK ('R');           /* PTR is read only */
 if ((r = attach_unit (uptr,cptr)) != SCPE_OK)
     return r;
 if ((sim_switches & SWMASK ('F')) ||
diff --git a/PDP1/pdp1_dt.c b/PDP1/pdp1_dt.c
index 856c2393..39478e80 100644
--- a/PDP1/pdp1_dt.c
+++ b/PDP1/pdp1_dt.c
@@ -1023,6 +1023,7 @@ else if (uptr->flags & UNIT_11FMT)
     sim_printf ("16b format");
 else sim_printf ("18b/36b format");
 sim_printf (", buffering file in memory\n");
+sim_fseek (uptr->fileref, 0, SEEK_SET);                 /* start at the beginning of file */
 if (uptr->flags & UNIT_8FMT) {                          /* 12b? */
     for (ba = 0; ba < uptr->capac; ) {                  /* loop thru file */
         k = fxread (pdp8b, sizeof (uint16), D8_NBSIZE, uptr->fileref);
diff --git a/PDP1/pdp1_stddev.c b/PDP1/pdp1_stddev.c
index 7af437be..f18eeea3 100644
--- a/PDP1/pdp1_stddev.c
+++ b/PDP1/pdp1_stddev.c
@@ -458,6 +458,7 @@ ptr_leader = PTR_LEADER;                                /* set up leader */
 if (sim_switches & SWMASK ('A'))
     uptr->flags = uptr->flags | UNIT_ASCII;
 else uptr->flags = uptr->flags & ~UNIT_ASCII;
+sim_switches |= SWMASK ('R');
 return attach_unit (uptr, cptr);
 }
 
diff --git a/PDP10/kx10_dt.c b/PDP10/kx10_dt.c
index 449e7889..0e82c56d 100644
--- a/PDP10/kx10_dt.c
+++ b/PDP10/kx10_dt.c
@@ -1182,6 +1182,7 @@ t_stat dt_attach (UNIT *uptr, CONST char *cptr)
         sim_printf ("16b format");
     else sim_printf ("18b/36b format");
     sim_printf (", buffering file in memory\n");
+    (void)sim_fseek (uptr->fileref, 0, SEEK_SET);           /* start at the beginning of file */
     uptr->io_flush = dt_flush;
     if (uptr->flags & UNIT_8FMT) {                          /* 12b? */
         for (ba = 0; ba < uptr->capac; ) {                  /* loop thru file */
diff --git a/PDP10/pdp6_dtc.c b/PDP10/pdp6_dtc.c
index c4e1567a..4ad93c93 100644
--- a/PDP10/pdp6_dtc.c
+++ b/PDP10/pdp6_dtc.c
@@ -1194,6 +1194,7 @@ dtc_attach (UNIT *uptr, CONST char *cptr)
         sim_printf ("16b format");
     else sim_printf ("18b/36b format");
     sim_printf (", buffering file in memory\n");
+    (void)sim_fseek (uptr->fileref, 0, SEEK_SET);           /* start at the beginning of file */
     uptr->WRITTEN = 0;
     uptr->io_flush = dtc_flush;
     if (uptr->flags & UNIT_8FMT) {                          /* 12b? */
diff --git a/PDP11/pdp11_tc.c b/PDP11/pdp11_tc.c
index 8012651a..5e27c756 100644
--- a/PDP11/pdp11_tc.c
+++ b/PDP11/pdp11_tc.c
@@ -1299,6 +1299,7 @@ else if (uptr->flags & UNIT_11FMT)
     sim_printf ("16b format");
 else sim_printf ("18b/36b format");
 sim_printf (", buffering file in memory\n");
+(void)sim_fseek (uptr->fileref, 0, SEEK_SET);           /* start at the beginning of file */
 uptr->io_flush = dt_flush;
 if (uptr->flags & UNIT_8FMT) {                          /* 12b? */
     for (ba = 0; ba < uptr->capac; ) {                  /* loop thru file */
diff --git a/PDP18B/pdp18b_dt.c b/PDP18B/pdp18b_dt.c
index 4b42a0ec..1c5922b1 100644
--- a/PDP18B/pdp18b_dt.c
+++ b/PDP18B/pdp18b_dt.c
@@ -1458,6 +1458,7 @@ else if (uptr->flags & UNIT_11FMT)
     sim_printf ("16b format");
 else sim_printf ("18b/36b format");
 sim_printf (", buffering file in memory\n");
+(void)sim_fseek (uptr->fileref, 0, SEEK_SET);           /* start at the beginning of file */
 uptr->io_flush = dt_flush;
 if (uptr->flags & UNIT_8FMT) {                          /* 12b? */
     for (ba = 0; ba < uptr->capac; ) {                  /* loop thru file */
diff --git a/PDP18B/pdp18b_stddev.c b/PDP18B/pdp18b_stddev.c
index 43dc3ce4..2dcc9f17 100644
--- a/PDP18B/pdp18b_stddev.c
+++ b/PDP18B/pdp18b_stddev.c
@@ -651,6 +651,7 @@ t_stat ptr_attach (UNIT *uptr, CONST char *cptr)
 {
 t_stat reason;
 
+sim_switches |= SWMASK ('R');
 reason = attach_unit (uptr, cptr);
 if (reason != SCPE_OK)
     return reason;
diff --git a/PDP8/pdp8_dt.c b/PDP8/pdp8_dt.c
index 7ac2330a..7d4f1e8c 100644
--- a/PDP8/pdp8_dt.c
+++ b/PDP8/pdp8_dt.c
@@ -1248,6 +1248,7 @@ else if (uptr->flags & UNIT_11FMT)
     sim_printf ("16b format");
 else sim_printf ("18b/36b format");
 sim_printf (", buffering file in memory\n");
+(void)sim_fseek (uptr->fileref, 0, SEEK_SET);           /* start at the beginning of file */
 uptr->io_flush = dt_flush;
 if (uptr->flags & UNIT_8FMT)                            /* 12b? */
     uptr->hwmark = fxread (uptr->filebuf, sizeof (uint16),
diff --git a/PDP8/pdp8_td.c b/PDP8/pdp8_td.c
index 7ae902f0..e01daff8 100644
--- a/PDP8/pdp8_td.c
+++ b/PDP8/pdp8_td.c
@@ -808,6 +808,7 @@ else if (uptr->flags & UNIT_11FMT)
     sim_printf ("16b format");
 else sim_printf ("18b/36b format");
 sim_printf (", buffering file in memory\n");
+(void)sim_fseek (uptr->fileref, 0, SEEK_SET);           /* start at the beginning of file */
 uptr->io_flush = td_flush;
 if (uptr->flags & UNIT_8FMT)                            /* 12b? */
     uptr->hwmark = fxread (uptr->filebuf, sizeof (uint16),