diff --git a/supervisor/session/plc.lua b/supervisor/session/plc.lua
index c36dbda..f799987 100644
--- a/supervisor/session/plc.lua
+++ b/supervisor/session/plc.lua
@@ -202,6 +202,12 @@ plc.new_session = function (id, for_reactor, in_queue, out_queue)
         self.sDB.mek_struct.max_burn  = mek_data[8]
     end
 
+    -- mark this PLC session as closed, stop watchdog
+    local _close = function ()
+        self.rtu_conn_watchdog.cancel()
+        self.connected = false
+    end
+
     -- send an RPLC packet
     ---@param msg_type RPLC_TYPES
     ---@param msg table
@@ -392,7 +398,7 @@ plc.new_session = function (id, for_reactor, in_queue, out_queue)
                 end
             elseif pkt.type == SCADA_MGMT_TYPES.CLOSE then
                 -- close the session
-                self.connected = false
+                _close()
             else
                 log.debug(log_header .. "handler received unsupported SCADA_MGMT packet type " .. pkt.type)
             end
@@ -427,13 +433,12 @@ plc.new_session = function (id, for_reactor, in_queue, out_queue)
 
     -- check if a timer matches this session's watchdog
     public.check_wd = function (timer)
-        return self.plc_conn_watchdog.is_timer(timer)
+        return self.plc_conn_watchdog.is_timer(timer) and self.connected
     end
 
     -- close the connection
     public.close = function ()
-        self.plc_conn_watchdog.cancel()
-        self.connected = false
+        _close()
         _send_mgmt(SCADA_MGMT_TYPES.CLOSE, {})
         println("connection to reactor " .. self.for_reactor .. " PLC closed by server")
         log.info(log_header .. "session closed by server")
@@ -506,7 +511,6 @@ plc.new_session = function (id, for_reactor, in_queue, out_queue)
 
             -- exit if connection was closed
             if not self.connected then
-                self.plc_conn_watchdog.cancel()
                 println("connection to reactor " .. self.for_reactor .. " PLC closed by remote host")
                 log.info(log_header .. "session closed by remote host")
                 return self.connected
diff --git a/supervisor/session/rtu.lua b/supervisor/session/rtu.lua
index 59cf0b1..33a66d7 100644
--- a/supervisor/session/rtu.lua
+++ b/supervisor/session/rtu.lua
@@ -129,6 +129,17 @@ rtu.new_session = function (id, in_queue, out_queue, advertisement)
         end
     end
 
+    -- mark this RTU session as closed, stop watchdog
+    local _close = function ()
+        self.rtu_conn_watchdog.cancel()
+        self.connected = false
+
+        -- mark all RTU unit sessions as closed so the reactor unit knows
+        for i = 1, #self.units do
+            self.units[i].close()
+        end
+    end
+
     -- send a SCADA management packet
     ---@param msg_type SCADA_MGMT_TYPES
     ---@param msg table
@@ -186,7 +197,7 @@ rtu.new_session = function (id, in_queue, out_queue, advertisement)
                 end
             elseif pkt.type == SCADA_MGMT_TYPES.CLOSE then
                 -- close the session
-                self.connected = false
+                _close()
             elseif pkt.type == SCADA_MGMT_TYPES.RTU_ADVERT then
                 -- RTU unit advertisement
                 -- handle advertisement; this will re-create all unit sub-sessions
@@ -206,13 +217,12 @@ rtu.new_session = function (id, in_queue, out_queue, advertisement)
     -- check if a timer matches this session's watchdog
     ---@param timer number
     public.check_wd = function (timer)
-        return self.rtu_conn_watchdog.is_timer(timer)
+        return self.rtu_conn_watchdog.is_timer(timer) and self.connected
     end
 
     -- close the connection
     public.close = function ()
-        self.rtu_conn_watchdog.cancel()
-        self.connected = false
+        _close()
         _send_mgmt(SCADA_MGMT_TYPES.CLOSE, {})
         println(log_header .. "connection to RTU closed by server")
         log.info(log_header .. "session closed by server")
@@ -272,7 +282,6 @@ rtu.new_session = function (id, in_queue, out_queue, advertisement)
 
             -- exit if connection was closed
             if not self.connected then
-                self.rtu_conn_watchdog.cancel()
                 println(log_header .. "connection to RTU closed by remote host")
                 log.info(log_header .. "session closed by remote host")
                 return self.connected
diff --git a/supervisor/session/rtu/boiler.lua b/supervisor/session/rtu/boiler.lua
index c0dd0e6..2b8047f 100644
--- a/supervisor/session/rtu/boiler.lua
+++ b/supervisor/session/rtu/boiler.lua
@@ -42,6 +42,7 @@ boiler.new = function (session_id, advert, out_queue)
         reactor = advert.reactor,
         out_q = out_queue,
         transaction_controller = txnctrl.new(),
+        connected = true,
         has_build = false,
         periodics = {
             next_build_req = 0,
@@ -180,6 +181,9 @@ boiler.new = function (session_id, advert, out_queue)
     public.get_reactor = function () return self.reactor end
     public.get_db = function () return self.db end
 
+    public.close = function () self.connected = false end
+    public.is_connected = function () return self.connected end
+
     -- update this runner
     ---@param time_now integer milliseconds
     public.update = function (time_now)
diff --git a/supervisor/session/rtu/emachine.lua b/supervisor/session/rtu/emachine.lua
index cdbf21e..8e2a2e8 100644
--- a/supervisor/session/rtu/emachine.lua
+++ b/supervisor/session/rtu/emachine.lua
@@ -41,6 +41,7 @@ emachine.new = function (session_id, advert, out_queue)
         reactor = 0,
         out_q = out_queue,
         transaction_controller = txnctrl.new(),
+        connected = true,
         has_build = false,
         periodics = {
             next_build_req = 0,
@@ -130,6 +131,9 @@ emachine.new = function (session_id, advert, out_queue)
     public.get_reactor = function () return self.reactor end
     public.get_db = function () return self.db end
 
+    public.close = function () self.connected = false end
+    public.is_connected = function () return self.connected end
+
     -- update this runner
     ---@param time_now integer milliseconds
     public.update = function (time_now)
diff --git a/supervisor/session/rtu/redstone.lua b/supervisor/session/rtu/redstone.lua
index 4380567..b5f44ea 100644
--- a/supervisor/session/rtu/redstone.lua
+++ b/supervisor/session/rtu/redstone.lua
@@ -60,6 +60,7 @@ redstone.new = function (session_id, advert, out_queue)
         in_q = mqueue.new(),
         out_q = out_queue,
         transaction_controller = txnctrl.new(),
+        connected = true,
         has_di = false,
         has_ai = false,
         periodics = {
@@ -192,6 +193,9 @@ redstone.new = function (session_id, advert, out_queue)
     public.get_reactor = function () return self.reactor end
     public.get_db = function () return self.db end
 
+    public.close = function () self.connected = false end
+    public.is_connected = function () return self.connected end
+
     -- update this runner
     ---@param time_now integer milliseconds
     public.update = function (time_now)
diff --git a/supervisor/session/rtu/turbine.lua b/supervisor/session/rtu/turbine.lua
index 292ff95..e6d41c2 100644
--- a/supervisor/session/rtu/turbine.lua
+++ b/supervisor/session/rtu/turbine.lua
@@ -43,6 +43,7 @@ turbine.new = function (session_id, advert, out_queue)
         reactor = advert.reactor,
         out_q = out_queue,
         transaction_controller = txnctrl.new(),
+        connected = true,
         has_build = false,
         periodics = {
             next_build_req = 0,
@@ -171,6 +172,9 @@ turbine.new = function (session_id, advert, out_queue)
     public.get_reactor = function () return self.reactor end
     public.get_db = function () return self.db end
 
+    public.close = function () self.connected = false end
+    public.is_connected = function () return self.connected end
+
     -- update this runner
     ---@param time_now integer milliseconds
     public.update = function (time_now)