diff --git a/VAX/vax750_cmi.c b/VAX/vax750_cmi.c
index 4b4416e2..02965612 100644
--- a/VAX/vax750_cmi.c
+++ b/VAX/vax750_cmi.c
@@ -585,6 +585,10 @@ static struct boot_dev boot_tab[] = {
     { "RP",  "DB",  "ka750_new.bin", BOOT_B, 0,        0        },  /* DBAn */
     { "RP",  "DRA", "ka750_new.bin", BOOT_B, BOOT_MB,  0        },  /* DRAn */
     { "RP",  "DR",  "ka750_new.bin", BOOT_B, 0,        0        },  /* DRAn */
+    { "RPB", "DBB", "ka750_new.bin", BOOT_B, BOOT_MB,  0        },  /* DBAn */
+    { "RPB", "DBB", "ka750_new.bin", BOOT_B, 0,        0        },  /* DBAn */
+    { "RPB", "DRB", "ka750_new.bin", BOOT_B, BOOT_MB,  0        },  /* DRAn */
+    { "RPB", "DRB", "ka750_new.bin", BOOT_B, 0,        0        },  /* DRAn */
     { "HK",  "DMA", "ka750_old.bin", BOOT_B, BOOT_HK,  0        },  /* DMAn */
     { "HK",  "DM",  "ka750_old.bin", BOOT_B, 0,        0        },  /* DMAn */
     { "RL",  "DLA", "ka750_old.bin", BOOT_C, BOOT_RL,  0        },  /* DLAn */
diff --git a/VAX/vax750_defs.h b/VAX/vax750_defs.h
index 4c7b451e..a7cdbbea 100644
--- a/VAX/vax750_defs.h
+++ b/VAX/vax750_defs.h
@@ -79,10 +79,11 @@
 
 #define NEXUS_NUM       16                              /* number of nexus */
 #define MCTL_NUM        2                               /* number of mem ctrl */
-#define MBA_NUM         2                               /* number of MBA's */
+#define MBA_NUM         3                               /* number of MBA's */
 #define TR_MCTL         0                               /* nexus assignments */
 #define TR_MBA0         4
 #define TR_MBA1         5
+#define TR_MBA2         6
 #define TR_UBA          8
 #define TR_CI           15
 #define NEXUS_HLVL      (IPL_HMAX - IPL_HMIN + 1)
@@ -99,6 +100,7 @@
 #define IPL_UBA         (0x15 - IPL_HMIN)
 #define IPL_MBA0        (0x15 - IPL_HMIN)
 #define IPL_MBA1        (0x15 - IPL_HMIN)
+#define IPL_MBA2        (0x15 - IPL_HMIN)
 #define IPL_CI          (0x15 - IPL_HMIN)
 
 /* Nexus interrupt macros */
diff --git a/VAX/vax750_syslist.c b/VAX/vax750_syslist.c
index 75642805..82413400 100644
--- a/VAX/vax750_syslist.c
+++ b/VAX/vax750_syslist.c
@@ -55,6 +55,7 @@ extern DEVICE rl_dev;
 extern DEVICE hk_dev;
 extern DEVICE rk_dev;
 extern DEVICE rp_dev;
+extern DEVICE rpb_dev;
 extern DEVICE ry_dev;
 extern DEVICE ts_dev;
 extern DEVICE tq_dev;
@@ -75,6 +76,7 @@ DEVICE *sim_devices[] = {
     &uba_dev,
     &mba_dev[0],
     &mba_dev[1],
+    &mba_dev[2],
     &clk_dev,
     &tmr_dev,
     &tti_dev,
@@ -87,6 +89,7 @@ DEVICE *sim_devices[] = {
     &cr_dev,
     &lpt_dev,
     &rp_dev,
+    &rpb_dev,
     &rl_dev,
     &hk_dev,
     &rk_dev,
diff --git a/VAX/vax7x0_mba.c b/VAX/vax7x0_mba.c
index 23eefa49..0dc96dc0 100644
--- a/VAX/vax7x0_mba.c
+++ b/VAX/vax7x0_mba.c
@@ -269,7 +269,7 @@ static t_stat (*mbregR[MBA_NUM])(int32 *dat, int32 ad, int32 md);
 static t_stat (*mbregW[MBA_NUM])(int32 dat, int32 ad, int32 md);
 static int32 (*mbabort[MBA_NUM])(void);
 
-static int32 mba_active = 0;    /* Number of active MBA's */
+static uint32 mba_active = 0;   /* Number of active MBA's */
 
 /* Massbus adapter data structures
 
@@ -324,6 +324,7 @@ REG mba1_reg[] = {
     { NULL }
     };
 
+#if MBA_NUM != 2
 DIB mba2_dib = { TR_MBA2, 0, &mba_rdreg, &mba_wrreg, 0, NVCL (MBA2) };
 
 UNIT mba2_unit = { UDATA (NULL, 0, 0) };
@@ -346,6 +347,7 @@ REG mba2_reg[] = {
     { FLDATAD (NEXINT, nexus_req[IPL_MBA2], TR_MBA2, "nexus interrupt request") },
     { NULL }
     };
+#endif
 
 DEBTAB mba_deb[] = {
     { "REGREAD", MBA_DEB_RRD },
@@ -376,6 +378,7 @@ DEVICE mba_dev[] = {
     &mba1_dib, DEV_NEXUS | DEV_DEBUG, 0,
     mba_deb, NULL, NULL, &mba_help, NULL, NULL, 
     &mba_description
+#if MBA_NUM != 2
     },
     {
     "MBA2", &mba2_unit, mba2_reg, mba2_mod,
@@ -385,6 +388,7 @@ DEVICE mba_dev[] = {
     &mba2_dib, DEV_NEXUS | DEV_DEBUG, 0,
     mba_deb, NULL, NULL, &mba_help, NULL, NULL, 
     &mba_description
+#endif
     }
     };
 
@@ -392,8 +396,8 @@ DEVICE mba_dev[] = {
 
 t_stat mba_rdreg (int32 *val, int32 pa, int32 lnt)
 {
-int32 mb, ofs, drv, rtype;
-uint32 t;
+int32 ofs, drv, rtype;
+uint32 mb, t;
 t_stat r;
 
 mb = NEXUS_GETNEX (pa) - TR_MBA0;                       /* get MBA */
@@ -410,7 +414,7 @@ if ((pa & 3) || (lnt != L_LONG)) {                      /* unaligned or not lw?
     return SCPE_OK;
     }
 #endif
-if (mb >= MBA_NUM)                                      /* valid? */
+if (mb >= mba_active)                                   /* valid? */
     return SCPE_NXM;
 rtype = MBA_RTYPE (pa);                                 /* get reg type */
 
@@ -491,8 +495,8 @@ return SCPE_OK;
 
 t_stat mba_wrreg (int32 val, int32 pa, int32 lnt)
 {
-int32 mb, ofs, drv, rtype;
-uint32 old_reg, old_sr;
+int32 ofs, drv, rtype;
+uint32 mb, old_reg, old_sr;
 t_stat r;
 t_bool cs1dt;
 
@@ -504,7 +508,7 @@ if ((pa & 3) || (lnt != L_LONG)) {                      /* unaligned or not lw?
 #endif
     return SCPE_OK;
     }
-if (mb >= MBA_NUM)                                      /* valid? */
+if (mb >= mba_active)                                   /* valid? */
     return SCPE_NXM;
 rtype = MBA_RTYPE (pa);                                 /* get reg type */
 
@@ -630,7 +634,7 @@ int32 mba_rdbufW (uint32 mb, int32 bc, uint16 *buf)
 int32 i, j, ba, mbc, pbc;
 uint32 pa, dat;
 
-if (mb >= MBA_NUM)                                      /* valid MBA? */
+if (mb >= mba_active)                                   /* valid MBA? */
     return 0;
 ba = mba_va[mb];                                        /* get virt addr */
 mbc = (MBABC_WR + 1) - mba_bc[mb];                      /* get Mbus bc */
@@ -679,7 +683,7 @@ int32 mba_wrbufW (uint32 mb, int32 bc, const uint16 *buf)
 int32 i, j, ba, mbc, pbc;
 uint32 pa, dat;
 
-if (mb >= MBA_NUM)                                      /* valid MBA? */
+if (mb >= mba_active)                                   /* valid MBA? */
     return 0;
 ba = mba_va[mb];                                        /* get virt addr */
 mbc = (MBABC_WR + 1) - mba_bc[mb];                      /* get Mbus bc */
@@ -729,7 +733,7 @@ int32 mba_chbufW (uint32 mb, int32 bc, uint16 *buf)
 int32 i, j, ba, mbc, pbc;
 uint32 pa, dat, cmp;
 
-if (mb >= MBA_NUM)                                      /* valid MBA? */
+if (mb >= mba_active)                                   /* valid MBA? */
     return 0;
 ba = mba_va[mb];                                        /* get virt addr */
 mbc = (MBABC_WR + 1) - mba_bc[mb];                      /* get Mbus bc */
@@ -811,7 +815,7 @@ return;
 
 int32 mba_get_bc (uint32 mb)
 {
-if (mb >= MBA_NUM)
+if (mb >= mba_active)
     return 0;
 return (MBABC_WR + 1) - mba_bc[mb];
 }
@@ -820,7 +824,7 @@ void mba_set_int (uint32 mb)
 {
 DIB *dibp;
 
-if (mb >= MBA_NUM)
+if (mb >= mba_active)
     return;
 dibp = (DIB *) mba_dev[mb].ctxt;
 if (dibp) {
@@ -834,7 +838,7 @@ void mba_clr_int (uint32 mb)
 {
 DIB *dibp;
 
-if (mb >= MBA_NUM)
+if (mb >= mba_active)
     return;
 dibp = (DIB *) mba_dev[mb].ctxt;
 if (dibp) {
@@ -848,7 +852,7 @@ void mba_upd_sr (uint32 set, uint32 clr, uint32 mb)
 {
 uint32 o_sr;
 
-if (mb >= MBA_NUM)
+if (mb >= mba_active)
     return;
 o_sr = mba_sr[mb];
 if (set & MBASR_ABORTS)
@@ -894,14 +898,23 @@ return build_dib_tab();
 
 t_stat mba_help (FILE *st, DEVICE *dptr, UNIT *uptr, int32 flag, const char *cptr)
 {
-fprintf (st, "Massbus Adapters (MBA0, MBA1)\n\n");
-fprintf (st, "The Massbus adapters (MBA0, MBA1) simulate RH780's.  RP disk drives get\n");
-fprintf (st, "assigned to MBA0 if it is enabled, and TU tape drives get assigned to\n");
-fprintf (st, "MBA1 if RP is enabled, and MBA0 otherwise.\n");
-fprint_set_help (st, dptr);
-fprint_show_help (st, dptr);
-fprint_reg_help (st, dptr);
-return SCPE_OK;
+const char *const text =
+/*567901234567890123456789012345678901234567890123456789012345678901234567890*/
+#if MBA_NUM == 3
+" Massbus Adapters (MBA0, MBA1, MBA2)\n\n"
+" The Massbus adapters (MBA0, MBA1, MBA2) simulate RH780's.  RP, RPB and TU\n"
+" get assigned to MBA0, MBA1 and MBA2 in that order if they are enabled.\n"
+#else
+" Massbus Adapters (MBA0, MBA1)\n\n"
+" The Massbus adapters (MBA0, MBA1) simulate RH780's.  RP disk drives get\n"
+" assigned to MBA0 if it is enabled, and TU tape drives get assigned to\n"
+" MBA1 if RP is enabled, and MBA0 otherwise.\n"
+#endif
+"1$Set commands\n"
+"1$Show commands\n"
+"1$Registers\n";
+
+return scp_help (st, dptr, uptr, flag, text, "");
 }
 
 const char *mba_description (DEVICE *dptr)
@@ -1004,11 +1017,9 @@ if ((mbregR[idx] && dibp->rd &&                         /* conflict? */
     (mbregW[idx] && dibp->wr &&
     (mbregW[idx] != dibp->wr)) ||
     (mbabort[idx] && dibp->ack[0] &&
-    (mbabort[idx] != dibp->ack[0]))) {
-        sim_printf ("Massbus %s assignment conflict at %d\n",
-                    sim_dname (dptr), dibp->ba);
-        return SCPE_STOP;
-        }
+    (mbabort[idx] != dibp->ack[0])))
+    return sim_messagef (SCPE_STOP, "Massbus %s assignment conflict at %d\n",
+                                    sim_dname (dptr), dibp->ba);
 if (dibp->rd)                                           /* set rd dispatch */
     mbregR[idx] = dibp->rd;
 if (dibp->wr)                                           /* set wr dispatch */
diff --git a/VAX/vax860_abus.c b/VAX/vax860_abus.c
index d0be2711..7dafcc96 100644
--- a/VAX/vax860_abus.c
+++ b/VAX/vax860_abus.c
@@ -93,6 +93,7 @@ char cpu_boot_cmd[CBUFSIZE]  = { 0 };                   /* boot command */
 
 static struct boot_dev boot_tab[] = {
     { "RP", BOOT_MB, 0 },
+    { "RPB", BOOT_MB, 0 },
     { "HK", BOOT_HK, 0 },
     { "RL", BOOT_RL, 0 },
     { "RQ", BOOT_UDA, 1 << 24 },
diff --git a/VAX/vax860_defs.h b/VAX/vax860_defs.h
index 7fab89f5..d3570ace 100644
--- a/VAX/vax860_defs.h
+++ b/VAX/vax860_defs.h
@@ -110,12 +110,13 @@
 
 #define NEXUS_NUM       16                              /* number of nexus */
 #define MCTL_NUM        2                               /* number of mem ctrl */
-#define MBA_NUM         2                               /* number of MBA's */
+#define MBA_NUM         3                               /* number of MBA's */
 #define TR_MCTL0        1                               /* nexus assignments */
 #define TR_MCTL1        2
 #define TR_UBA          3
 #define TR_MBA0         8
 #define TR_MBA1         9
+#define TR_MBA2         10
 #define TR_CI           14
 #define NEXUS_HLVL      (IPL_HMAX - IPL_HMIN + 1)
 #define SCB_NEXUS       0x100                           /* nexus intr base */
@@ -131,6 +132,7 @@
 #define IPL_UBA         (0x15 - IPL_HMIN)
 #define IPL_MBA0        (0x15 - IPL_HMIN)
 #define IPL_MBA1        (0x15 - IPL_HMIN)
+#define IPL_MBA2        (0x15 - IPL_HMIN)
 #define IPL_CI          (0x15 - IPL_HMIN)
 
 /* Nexus interrupt macros */
diff --git a/VAX/vax860_syslist.c b/VAX/vax860_syslist.c
index c02520f4..6722b38d 100644
--- a/VAX/vax860_syslist.c
+++ b/VAX/vax860_syslist.c
@@ -55,6 +55,7 @@ extern DEVICE rl_dev;
 extern DEVICE hk_dev;
 extern DEVICE rk_dev;
 extern DEVICE rp_dev;
+extern DEVICE rpb_dev;
 extern DEVICE ry_dev;
 extern DEVICE ts_dev;
 extern DEVICE tq_dev;
@@ -74,6 +75,7 @@ DEVICE *sim_devices[] = {
     &uba_dev,
     &mba_dev[0],
     &mba_dev[1],
+    &mba_dev[2],
     &clk_dev,
     &tmr_dev,
     &tti_dev,
@@ -86,6 +88,7 @@ DEVICE *sim_devices[] = {
     &cr_dev,
     &lpt_dev,
     &rp_dev,
+    &rpb_dev,
     &rl_dev,
     &hk_dev,
     &rk_dev,
diff --git a/Visual Studio Projects/VAX750.vcproj b/Visual Studio Projects/VAX750.vcproj
index a14eeac5..1e32a0c0 100644
--- a/Visual Studio Projects/VAX750.vcproj	
+++ b/Visual Studio Projects/VAX750.vcproj	
@@ -242,6 +242,10 @@
 				RelativePath="..\PDP11\pdp11_rp.c"
 				>
 			</File>
+			<File
+				RelativePath="..\PDP11\pdp11_rpb.c"
+				>
+			</File>
 			<File
 				RelativePath="..\PDP11\pdp11_rq.c"
 				>
diff --git a/Visual Studio Projects/VAX780.vcproj b/Visual Studio Projects/VAX780.vcproj
index 19bb0f68..b963c86d 100644
--- a/Visual Studio Projects/VAX780.vcproj	
+++ b/Visual Studio Projects/VAX780.vcproj	
@@ -246,6 +246,10 @@
 				RelativePath="..\PDP11\pdp11_rp.c"
 				>
 			</File>
+			<File
+				RelativePath="..\PDP11\pdp11_rpb.c"
+				>
+			</File>
 			<File
 				RelativePath="..\PDP11\pdp11_rq.c"
 				>
diff --git a/Visual Studio Projects/VAX8600.vcproj b/Visual Studio Projects/VAX8600.vcproj
index 88ddfab1..a534a8e3 100644
--- a/Visual Studio Projects/VAX8600.vcproj	
+++ b/Visual Studio Projects/VAX8600.vcproj	
@@ -246,6 +246,10 @@
 				RelativePath="..\PDP11\pdp11_rp.c"
 				>
 			</File>
+			<File
+				RelativePath="..\PDP11\pdp11_rpb.c"
+				>
+			</File>
 			<File
 				RelativePath="..\PDP11\pdp11_rq.c"
 				>
diff --git a/makefile b/makefile
index 1f318380..471b48c9 100644
--- a/makefile
+++ b/makefile
@@ -1779,10 +1779,10 @@ VAX750 = ${VAXD}/vax_cpu.c ${VAXD}/vax_cpu1.c ${VAXD}/vax_fpa.c \
 	${PDP11D}/pdp11_rl.c ${PDP11D}/pdp11_rq.c ${PDP11D}/pdp11_ts.c \
 	${PDP11D}/pdp11_dz.c ${PDP11D}/pdp11_lp.c ${PDP11D}/pdp11_tq.c \
 	${PDP11D}/pdp11_xu.c ${PDP11D}/pdp11_ry.c ${PDP11D}/pdp11_cr.c \
-	${PDP11D}/pdp11_hk.c ${PDP11D}/pdp11_rp.c ${PDP11D}/pdp11_tu.c \
-	${PDP11D}/pdp11_vh.c ${PDP11D}/pdp11_dmc.c ${PDP11D}/pdp11_dup.c \
-	${PDP11D}/pdp11_td.c ${PDP11D}/pdp11_tc.c ${PDP11D}/pdp11_rk.c \
-	${PDP11D}/pdp11_io_lib.c ${PDP11D}/pdp11_ch.c \
+	${PDP11D}/pdp11_hk.c ${PDP11D}/pdp11_rp.c ${PDP11D}/pdp11_rpb.c \
+	${PDP11D}/pdp11_tu.c ${PDP11D}/pdp11_vh.c ${PDP11D}/pdp11_dmc.c \
+	${PDP11D}/pdp11_dup.c ${PDP11D}/pdp11_td.c ${PDP11D}/pdp11_tc.c \
+	${PDP11D}/pdp11_rk.c ${PDP11D}/pdp11_io_lib.c ${PDP11D}/pdp11_ch.c \
 	$(NETWORK_DEPS)
 VAX750_OPT = -DVM_VAX -DVAX_750 -DUSE_INT64 -DUSE_ADDR64 -I ${VAXD} -I ${PDP11D} ${NETWORK_OPT}
 
@@ -1829,10 +1829,10 @@ VAX8600 = ${VAXD}/vax_cpu.c ${VAXD}/vax_cpu1.c ${VAXD}/vax_fpa.c \
 	${PDP11D}/pdp11_rl.c ${PDP11D}/pdp11_rq.c ${PDP11D}/pdp11_ts.c \
 	${PDP11D}/pdp11_dz.c ${PDP11D}/pdp11_lp.c ${PDP11D}/pdp11_tq.c \
 	${PDP11D}/pdp11_xu.c ${PDP11D}/pdp11_ry.c ${PDP11D}/pdp11_cr.c \
-	${PDP11D}/pdp11_rp.c ${PDP11D}/pdp11_tu.c ${PDP11D}/pdp11_hk.c \
-	${PDP11D}/pdp11_vh.c ${PDP11D}/pdp11_dmc.c ${PDP11D}/pdp11_dup.c \
-	${PDP11D}/pdp11_td.c ${PDP11D}/pdp11_tc.c ${PDP11D}/pdp11_rk.c \
-	${PDP11D}/pdp11_io_lib.c ${PDP11D}/pdp11_ch.c \
+	${PDP11D}/pdp11_rp.c ${PDP11D}/pdp11_rpb.c ${PDP11D}/pdp11_tu.c \
+	${PDP11D}/pdp11_hk.c ${PDP11D}/pdp11_vh.c ${PDP11D}/pdp11_dmc.c \
+	${PDP11D}/pdp11_dup.c ${PDP11D}/pdp11_td.c ${PDP11D}/pdp11_tc.c \
+	${PDP11D}/pdp11_rk.c ${PDP11D}/pdp11_io_lib.c ${PDP11D}/pdp11_ch.c \
 	$(NETWORK_DEPS)
 VAX8600_OPT = -DVM_VAX -DVAX_860 -DUSE_INT64 -DUSE_ADDR64 -I ${VAXD} -I ${PDP11D} ${NETWORK_OPT}