diff --git a/reactor-plc/plc.lua b/reactor-plc/plc.lua
index 0d13091..efe6aaa 100644
--- a/reactor-plc/plc.lua
+++ b/reactor-plc/plc.lua
@@ -778,6 +778,7 @@ function plc.comms(id, version, modem, plc_channel, svr_channel, range, reactor,
         -- print a log message to the terminal as long as the UI isn't running
         local function println_ts(message) if not plc_state.fp_ok then util.println_ts(message) end end
 
+        local protocol = packet.scada_frame.protocol()
         local l_chan   = packet.scada_frame.local_channel()
         local src_addr = packet.scada_frame.src_addr()
 
@@ -789,6 +790,9 @@ function plc.comms(id, version, modem, plc_channel, svr_channel, range, reactor,
             elseif self.linked and ((self.r_seq_num + 1) ~= packet.scada_frame.seq_num()) then
                 log.warning("sequence out-of-order: last = " .. self.r_seq_num .. ", new = " .. packet.scada_frame.seq_num())
                 return
+            elseif self.linked and src_addr ~= self.sv_addr then
+                log.debug("received packet from unknown computer " .. src_addr .. " while linked; channel in use by another system?")
+                return
             else
                 self.r_seq_num = packet.scada_frame.seq_num()
             end
@@ -796,8 +800,6 @@ function plc.comms(id, version, modem, plc_channel, svr_channel, range, reactor,
             -- feed the watchdog first so it doesn't uhh...eat our packets :)
             conn_watchdog.feed()
 
-            local protocol = packet.scada_frame.protocol()
-
             -- handle packet
             if protocol == PROTOCOL.RPLC then
                 ---@cast packet rplc_frame
diff --git a/reactor-plc/startup.lua b/reactor-plc/startup.lua
index 71ebcb2..5d0234f 100644
--- a/reactor-plc/startup.lua
+++ b/reactor-plc/startup.lua
@@ -18,7 +18,7 @@ local plc      = require("reactor-plc.plc")
 local renderer = require("reactor-plc.renderer")
 local threads  = require("reactor-plc.threads")
 
-local R_PLC_VERSION = "v1.4.2"
+local R_PLC_VERSION = "v1.4.3"
 
 local println = util.println
 local println_ts = util.println_ts
diff --git a/rtu/rtu.lua b/rtu/rtu.lua
index 5d2e2c8..a1cc9dc 100644
--- a/rtu/rtu.lua
+++ b/rtu/rtu.lua
@@ -330,13 +330,20 @@ function rtu.comms(version, modem, rtu_channel, svr_channel, range, conn_watchdo
         -- print a log message to the terminal as long as the UI isn't running
         local function println_ts(message) if not rtu_state.fp_ok then util.println_ts(message) end end
 
-        if packet.scada_frame.local_channel() == rtu_channel then
+        local protocol = packet.scada_frame.protocol()
+        local l_chan   = packet.scada_frame.local_channel()
+        local src_addr = packet.scada_frame.src_addr()
+
+        if l_chan == rtu_channel then
             -- check sequence number
             if self.r_seq_num == nil then
                 self.r_seq_num = packet.scada_frame.seq_num()
             elseif rtu_state.linked and ((self.r_seq_num + 1) ~= packet.scada_frame.seq_num()) then
                 log.warning("sequence out-of-order: last = " .. self.r_seq_num .. ", new = " .. packet.scada_frame.seq_num())
                 return
+            elseif rtu_state.linked and src_addr ~= self.sv_addr then
+                log.debug("received packet from unknown computer " .. src_addr .. " while linked; channel in use by another system?")
+                return
             else
                 self.r_seq_num = packet.scada_frame.seq_num()
             end
@@ -344,8 +351,7 @@ function rtu.comms(version, modem, rtu_channel, svr_channel, range, conn_watchdo
             -- feed watchdog on valid sequence number
             conn_watchdog.feed()
 
-            local protocol = packet.scada_frame.protocol()
-
+            -- handle packet
             if protocol == PROTOCOL.MODBUS_TCP then
                 ---@cast packet modbus_frame
                 if rtu_state.linked then
@@ -465,6 +471,8 @@ function rtu.comms(version, modem, rtu_channel, svr_channel, range, conn_watchdo
                 -- should be unreachable assuming packet is from parse_packet()
                 log.error("illegal packet type " .. protocol, true)
             end
+        else
+            log.debug("received packet on unconfigured channel " .. l_chan, true)
         end
     end
 
diff --git a/rtu/startup.lua b/rtu/startup.lua
index 113b513..58970f7 100644
--- a/rtu/startup.lua
+++ b/rtu/startup.lua
@@ -28,7 +28,7 @@ local sna_rtu      = require("rtu.dev.sna_rtu")
 local sps_rtu      = require("rtu.dev.sps_rtu")
 local turbinev_rtu = require("rtu.dev.turbinev_rtu")
 
-local RTU_VERSION = "v1.3.2"
+local RTU_VERSION = "v1.3.3"
 
 local RTU_UNIT_TYPE = types.RTU_UNIT_TYPE
 local RTU_UNIT_HW_STATE = databus.RTU_UNIT_HW_STATE