diff --git a/PDP11/pdp11_tc.c b/PDP11/pdp11_tc.c
index d424c9d6..fe9a2f5e 100644
--- a/PDP11/pdp11_tc.c
+++ b/PDP11/pdp11_tc.c
@@ -1,6 +1,6 @@
 /* pdp11_tc.c: PDP-11 DECtape simulator
 
-   Copyright (c) 1993-2013, Robert M Supnik
+   Copyright (c) 1993-2016, Robert M Supnik
 
    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
@@ -25,6 +25,7 @@
 
    tc           TC11/TU56 DECtape
 
+   04-Dec-13    RMS     Revised to model TCCM correctly (Josh Dersch)
    23-Oct-13    RMS     Revised for new boot setup routine
    23-Jun-06    RMS     Fixed switch conflict in ATTACH
    10-Feb-06    RMS     READ sets extended data bits in TCST (Alan Frisbie)
@@ -274,11 +275,11 @@ extern int32 int_req[IPL_HLVL];
 #define LOG_RW          0x2
 #define LOG_BL          0x4
 
-#define DT_SETDONE      tccm = tccm | CSR_DONE; \
-                        if (tccm & CSR_IE) \
-                            SET_INT (DTA)
-#define DT_CLRDONE      tccm = tccm & ~CSR_DONE; \
-                        CLR_INT (DTA)
+#define DT_SETDONE      if ((tccm & (CSR_DONE|CSR_IE)) == CSR_IE) \
+                            SET_INT (DTA); \
+                        tccm = tccm | CSR_DONE
+#define DT_CLRDONE      CLR_INT (DTA); \
+                        tccm = tccm & ~CSR_DONE
 #define ABS(x)          (((x) < 0)? (-(x)): (x))
 
 int32 tcst = 0;                                         /* status */
@@ -411,7 +412,14 @@ DEVICE dt_dev = {
     &dt_description
     };
 
-/* IO dispatch routines, I/O addresses 17777340 - 17777350 */
+/* IO dispatch routines, I/O addresses 17777340 - 17777350
+
+   Read hardware notes:
+
+   - While the TCCM error bit is a real flop, it is supposed to reflect
+     the OR of the TCST error bits at all time, so it is updated on read.
+   - A read of TCDT while the function is RALL clears DONE.
+*/
 
 t_stat dt_rd (int32 *data, int32 PA, int32 access)
 {
@@ -456,6 +464,28 @@ switch (j) {
 return SCPE_OK;
 }
 
+/* Write hardware notes:
+
+   - The TC11 behaves much more like a traditional DECtape controller
+     than a typical PDP11 peripheral. In particular, execution is
+     initiated/controlled by any write to TCCM, rather than setting
+     the GO (DO) bit. Unless the function is STOP or STOP ALL, writing
+     TCCM will put the selected tape in motion.
+   - Writing GO (DO) clears DONE (READY) and the error flops in TCST.
+   - Writing a 0 to ERROR clears the error flops in TCST. Because it
+     is write 0 to clear (later controllers used write 1 to clear),
+     the simulator has to know whether ERROR is actually written.
+   - STOP ALL ignores select errors. Every other function is rejected
+     if there is a select error.
+   - An illegal operation (setting ILO) will stop the selected tape.
+   - A write of TCDT while the function is RALL, WALL, or WTMK clears
+     DONE (READY). RALL should not be included, but it saved a gate
+     not to prevent it.
+   - Because DONE (READY) may not be clear when an operation completes
+     and DONE (READY) is set, the DT_SETDONE must test for DONE (READY)
+     not being already set.
+*/
+
 t_stat dt_wr (int32 data, int32 PA, int32 access)
 {
 int32 i, j, unum, old_tccm, fnc;
@@ -472,43 +502,45 @@ switch (j) {
 
     case 001:                                           /* TCCM */
         old_tccm = tccm;                                /* save prior */
-        if (access == WRITEB)
-            data = (PA & 1)? (tccm & 0377) | (data << 8): (tccm & ~0377) | data;
-        if ((data & CSR_IE) == 0)
-            CLR_INT (DTA);
-        else if ((((tccm & CSR_IE) == 0) && (tccm & CSR_DONE)) ||
-            (data & CSR_DONE)) SET_INT (DTA);
-        tccm = (tccm & ~CSR_RW) | (data & CSR_RW);
-        if ((data & CSR_GO) && (tccm & CSR_DONE)) {     /* new cmd? */
+        if (access == WRITEB)                
+            data = (PA & 1)? ((tccm & 0377) | (data << 8)): ((tccm & ~0377) | data);
+        if ((data & CSR_IE) == 0)                       /* clearing IE? */
+            CLR_INT (DTA);                              /* clear intr */
+        else if ((tccm & (CSR_DONE|CSR_IE)) == CSR_DONE) /* setting, DON'IE = DON? */
+            SET_INT (DTA);                              /* set intr */
+        tccm = (tccm & ~CSR_RW) | (data & CSR_RW);      /* merge data */
+        if ((data & CSR_GO) != 0) {                     /* GO (DO) set? */
             tcst = tcst & ~STA_ALLERR;                  /* clear errors */
-            tccm = tccm & ~(CSR_ERR | CSR_DONE);        /* clear done, err */
-            CLR_INT (DTA);                              /* clear int */
-            if ((old_tccm ^ tccm) & CSR_UNIT)
-                dt_deselect (old_tccm);
-            unum = CSR_GETUNIT (tccm);                  /* get drive */
-            fnc = CSR_GETFNC (tccm);                    /* get function */
-            if (fnc == FNC_STOP) {                      /* stop all? */
-                sim_activate (&dt_dev.units[DT_TIMER], dt_ctime);
-                for (i = 0; i < DT_NUMDR; i++)
-                    dt_stopunit (dt_dev.units + i);     /* stop unit */
-                break;
-                }
-            uptr = dt_dev.units + unum;
-            if (uptr->flags & UNIT_DIS)                 /* disabled? */
-                dt_seterr (uptr, STA_SEL);              /* select err */
-            if ((fnc == FNC_WMRK) ||                    /* write mark? */
-                ((fnc == FNC_WALL) && (uptr->flags & UNIT_WPRT)) ||
-                ((fnc == FNC_WRIT) && (uptr->flags & UNIT_WPRT)))
-                dt_seterr (uptr, STA_ILO);              /* illegal op */
-            if (!(tccm & CSR_ERR))
-                dt_newsa (tccm);
+            tccm = tccm & ~(CSR_ERR|CSR_DONE);          /* clear done, err flops */
             }
-        else if ((tccm & CSR_ERR) == 0) {               /* clear err? */
-            tcst = tcst & ~STA_RWERR;
-            if (tcst & STA_ALLERR)
-                tccm = tccm | CSR_ERR;
+        else if (((data & CSR_ERR) == 0) &&             /* error bit clear? */
+            ((access != WRITEB) || ((PA & 1) != 0))) {  /* not write low byte? */
+            tcst = tcst & ~STA_ALLERR;                  /* clear errors */
+            tccm = tccm & ~CSR_ERR;                     /* clear err flop */
             }
-        break;
+        if (((old_tccm ^ tccm) & CSR_UNIT) != 0)        /* unit change? */
+            dt_deselect (old_tccm);                     /* deselect all */
+        unum = CSR_GETUNIT (tccm);                      /* get drive */
+        fnc = CSR_GETFNC (tccm);                        /* get function */
+        if (fnc == FNC_STOP) {                          /* stop all? */
+            sim_activate (&dt_dev.units[DT_TIMER], dt_ctime); /* sched done */
+            for (i = 0; i < DT_NUMDR; i++)              /* loop thru units */
+                dt_stopunit (dt_dev.units + i);         /* stop unit */
+            break;
+            }
+        uptr = dt_dev.units + unum;
+        if (uptr->flags & UNIT_DIS)                     /* disabled? */
+            dt_seterr (uptr, STA_SEL);                  /* select err */
+        if ((fnc == FNC_WMRK) ||                        /* write mark? */
+            ((fnc == FNC_WALL) && (uptr->flags & UNIT_WPRT)) ||
+            ((fnc == FNC_WRIT) && (uptr->flags & UNIT_WPRT)))
+            dt_seterr (uptr, STA_ILO);                  /* illegal op */
+        if ((tccm & CSR_ERR) != 0) {                    /* error? */
+            dt_stopunit (uptr);                         /* stop the unit */
+            DT_SETDONE;                                 /* set done at once */
+            }
+         else dt_newsa (tccm);                          /* new function */
+         break;
 
     case 002:                                           /* TCWC */
         tcwc = data;                                    /* word write only! */