#47 RTU luadoc, bugfixes

rtu/modbus.lua

@@ -7,14 +7,22 @@ local MODBUS_FCODE = types.MODBUS_FCODE
 local MODBUS_EXCODE = types.MODBUS_EXCODE
 
 -- new modbus comms handler object
+---@param rtu_dev rtu RTU device
+---@param use_parallel_read boolean whether or not to use parallel calls when reading
 modbus.new = function (rtu_dev, use_parallel_read)
     local self = {
         rtu = rtu_dev,
         use_parallel = use_parallel_read
     }
 
+    ---@class modbus
+    local public = {}
+
     local insert = table.insert
 
+    ---@param c_addr_start integer
+    ---@param count integer
+    ---@return boolean ok, table readings
     local _1_read_coils = function (c_addr_start, count)
         local tasks = {}
         local readings = {}
@@ -58,6 +66,9 @@ modbus.new = function (rtu_dev, use_parallel_read)
         return return_ok, readings
     end
 
+    ---@param di_addr_start integer
+    ---@param count integer
+    ---@return boolean ok, table readings
     local _2_read_discrete_inputs = function (di_addr_start, count)
         local tasks = {}
         local readings = {}
@@ -101,6 +112,9 @@ modbus.new = function (rtu_dev, use_parallel_read)
         return return_ok, readings
     end
 
+    ---@param hr_addr_start integer
+    ---@param count integer
+    ---@return boolean ok, table readings
     local _3_read_multiple_holding_registers = function (hr_addr_start, count)
         local tasks = {}
         local readings = {}
@@ -144,6 +158,9 @@ modbus.new = function (rtu_dev, use_parallel_read)
         return return_ok, readings
     end
 
+    ---@param ir_addr_start integer
+    ---@param count integer
+    ---@return boolean ok, table readings
     local _4_read_input_registers = function (ir_addr_start, count)
         local tasks = {}
         local readings = {}
@@ -187,6 +204,9 @@ modbus.new = function (rtu_dev, use_parallel_read)
         return return_ok, readings
     end
 
+    ---@param c_addr integer
+    ---@param value any
+    ---@return boolean ok, MODBUS_EXCODE|nil
     local _5_write_single_coil = function (c_addr, value)
         local response = nil
         local _, coils, _, _ = self.rtu.io_count()
@@ -206,6 +226,9 @@ modbus.new = function (rtu_dev, use_parallel_read)
         return return_ok, response
     end
 
+    ---@param hr_addr integer
+    ---@param value any
+    ---@return boolean ok, MODBUS_EXCODE|nil
     local _6_write_single_holding_register = function (hr_addr, value)
         local response = nil
         local _, _, _, hold_regs = self.rtu.io_count()
@@ -222,9 +245,12 @@ modbus.new = function (rtu_dev, use_parallel_read)
             response = MODBUS_EXCODE.ILLEGAL_DATA_ADDR
         end
 
-        return return_ok
+        return return_ok, response
     end
 
+    ---@param c_addr_start integer
+    ---@param values any
+    ---@return boolean ok, MODBUS_EXCODE|nil
     local _15_write_multiple_coils = function (c_addr_start, values)
         local response = nil
         local _, coils, _, _ = self.rtu.io_count()
@@ -249,6 +275,9 @@ modbus.new = function (rtu_dev, use_parallel_read)
         return return_ok, response
     end
 
+    ---@param hr_addr_start integer
+    ---@param values any
+    ---@return boolean ok, MODBUS_EXCODE|nil
     local _16_write_multiple_holding_registers = function (hr_addr_start, values)
         local response = nil
         local _, _, _, hold_regs = self.rtu.io_count()
@@ -274,7 +303,9 @@ modbus.new = function (rtu_dev, use_parallel_read)
     end
 
     -- validate a request without actually executing it
-    local check_request = function (packet)
+    ---@param packet modbus_frame
+    ---@return boolean return_code, modbus_packet reply
+    public.check_request = function (packet)
         local return_code = true
         local response = { MODBUS_EXCODE.ACKNOWLEDGE }
 
@@ -314,7 +345,9 @@ modbus.new = function (rtu_dev, use_parallel_read)
     end
 
     -- handle a MODBUS TCP packet and generate a reply
-    local handle_packet = function (packet)
+    ---@param packet modbus_frame
+    ---@return boolean return_code, modbus_packet reply
+    public.handle_packet = function (packet)
         local return_code = true
         local response = nil
 
@@ -369,7 +402,8 @@ modbus.new = function (rtu_dev, use_parallel_read)
     end
 
     -- return a SERVER_DEVICE_BUSY error reply
-    local reply__srv_device_busy = function (packet)
+    ---@return modbus_packet reply
+    public.reply__srv_device_busy = function (packet)
         -- reply back with error flag and exception code
         local reply = comms.modbus_packet()
         local fcode = bit.bor(packet.func_code, MODBUS_FCODE.ERROR_FLAG)
@@ -379,7 +413,8 @@ modbus.new = function (rtu_dev, use_parallel_read)
     end
 
     -- return a NEG_ACKNOWLEDGE error reply
-    local reply__neg_ack = function (packet)
+    ---@return modbus_packet reply
+    public.reply__neg_ack = function (packet)
         -- reply back with error flag and exception code
         local reply = comms.modbus_packet()
         local fcode = bit.bor(packet.func_code, MODBUS_FCODE.ERROR_FLAG)
@@ -389,7 +424,8 @@ modbus.new = function (rtu_dev, use_parallel_read)
     end
 
     -- return a GATEWAY_PATH_UNAVAILABLE error reply
-    local reply__gw_unavailable = function (packet)
+    ---@return modbus_packet reply
+    public.reply__gw_unavailable = function (packet)
         -- reply back with error flag and exception code
         local reply = comms.modbus_packet()
         local fcode = bit.bor(packet.func_code, MODBUS_FCODE.ERROR_FLAG)
@@ -398,13 +434,7 @@ modbus.new = function (rtu_dev, use_parallel_read)
         return reply
     end
 
-    return {
-        check_request = check_request,
-        handle_packet = handle_packet,
-        reply__srv_device_busy = reply__srv_device_busy,
-        reply__neg_ack = reply__neg_ack,
-        reply__gw_unavailable = reply__gw_unavailable
-    }
+    return public
 end
 
 return modbus

rtu/rtu.lua

@@ -19,6 +19,7 @@ local println = util.println
 local print_ts = util.print_ts
 local println_ts = util.println_ts
 
+-- create a new RTU
 rtu.init_unit = function ()
     local self = {
         discrete_inputs = {},
@@ -28,28 +29,36 @@ rtu.init_unit = function ()
         io_count_cache = { 0, 0, 0, 0 }
     }
 
+    ---@class rtu
+    local public = {}
+
     local insert = table.insert
 
     local _count_io = function ()
         self.io_count_cache = { #self.discrete_inputs, #self.coils, #self.input_regs, #self.holding_regs }
     end
 
-    -- return : IO count table
-    local io_count = function ()
+    -- return IO counts
+    ---@return integer discrete_inputs, integer coils, integer input_regs, integer holding_regs
+    public.io_count = function ()
         return self.io_count_cache[0], self.io_count_cache[1], self.io_count_cache[2], self.io_count_cache[3]
     end
 
     -- discrete inputs: single bit read-only
 
-    -- return : count of discrete inputs
-    local connect_di = function (f)
+    -- connect discrete input
+    ---@param f function
+    ---@return integer count count of discrete inputs
+    public.connect_di = function (f)
         insert(self.discrete_inputs, f)
         _count_io()
         return #self.discrete_inputs
     end
 
-    -- return : value, access fault
-    local read_di = function (di_addr)
+    -- read discrete input
+    ---@param di_addr integer
+    ---@return any value, boolean access_fault
+    public.read_di = function (di_addr)
         ppm.clear_fault()
         local value = self.discrete_inputs[di_addr]()
         return value, ppm.is_faulted()
@@ -57,22 +66,30 @@ rtu.init_unit = function ()
 
     -- coils: single bit read-write
 
-    -- return : count of coils
-    local connect_coil = function (f_read, f_write)
+    -- connect coil
+    ---@param f_read function
+    ---@param f_write function
+    ---@return integer count count of coils
+    public.connect_coil = function (f_read, f_write)
         insert(self.coils, { read = f_read, write = f_write })
         _count_io()
         return #self.coils
     end
 
-    -- return : value, access fault
-    local read_coil = function (coil_addr)
+    -- read coil
+    ---@param coil_addr integer
+    ---@return any value, boolean access_fault
+    public.read_coil = function (coil_addr)
         ppm.clear_fault()
         local value = self.coils[coil_addr].read()
         return value, ppm.is_faulted()
     end
 
-    -- return : access fault
-    local write_coil = function (coil_addr, value)
+    -- write coil
+    ---@param coil_addr integer
+    ---@param value any
+    ---@return boolean access_fault
+    public.write_coil = function (coil_addr, value)
         ppm.clear_fault()
         self.coils[coil_addr].write(value)
         return ppm.is_faulted()
@@ -80,15 +97,19 @@ rtu.init_unit = function ()
 
     -- input registers: multi-bit read-only
 
-    -- return : count of input registers
-    local connect_input_reg = function (f)
+    -- connect input register
+    ---@param f function
+    ---@return integer count count of input registers
+    public.connect_input_reg = function (f)
         insert(self.input_regs, f)
         _count_io()
         return #self.input_regs
     end
 
-    -- return : value, access fault
-    local read_input_reg = function (reg_addr)
+    -- read input register
+    ---@param reg_addr integer
+    ---@return any value, boolean access_fault
+    public.read_input_reg = function (reg_addr)
         ppm.clear_fault()
         local value = self.coils[reg_addr]()
         return value, ppm.is_faulted()
@@ -96,42 +117,43 @@ rtu.init_unit = function ()
 
     -- holding registers: multi-bit read-write
 
-    -- return : count of holding registers
-    local connect_holding_reg = function (f_read, f_write)
+    -- connect holding register
+    ---@param f_read function
+    ---@param f_write function
+    ---@return integer count count of holding registers
+    public.connect_holding_reg = function (f_read, f_write)
         insert(self.holding_regs, { read = f_read, write = f_write })
         _count_io()
         return #self.holding_regs
     end
 
-    -- return : value, access fault
-    local read_holding_reg = function (reg_addr)
+    -- read holding register
+    ---@param reg_addr integer
+    ---@return any value, boolean access_fault
+    public.read_holding_reg = function (reg_addr)
         ppm.clear_fault()
         local value = self.coils[reg_addr].read()
         return value, ppm.is_faulted()
     end
 
-    -- return : access fault
-    local write_holding_reg = function (reg_addr, value)
+    -- write holding register
+    ---@param reg_addr integer
+    ---@param value any
+    ---@return boolean access_fault
+    public.write_holding_reg = function (reg_addr, value)
         ppm.clear_fault()
         self.coils[reg_addr].write(value)
         return ppm.is_faulted()
     end
 
-    return {
-        io_count = io_count,
-        connect_di = connect_di,
-        read_di = read_di,
-        connect_coil = connect_coil,
-        read_coil = read_coil,
-        write_coil = write_coil,
-        connect_input_reg = connect_input_reg,
-        read_input_reg = read_input_reg,
-        connect_holding_reg = connect_holding_reg,
-        read_holding_reg = read_holding_reg,
-        write_holding_reg = write_holding_reg
-    }
+    return public
 end
 
+-- RTU Communications
+---@param modem table
+---@param local_port integer
+---@param server_port integer
+---@param conn_watchdog watchdog
 rtu.comms = function (modem, local_port, server_port, conn_watchdog)
     local self = {
         seq_num = 0,
@@ -143,6 +165,9 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
         conn_watchdog = conn_watchdog
     }
 
+    ---@class rtu_comms
+    local public = {}
+
     local insert = table.insert
 
     -- open modem
@@ -152,6 +177,9 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
 
     -- PRIVATE FUNCTIONS --
 
+    -- send a scada management packet
+    ---@param msg_type SCADA_MGMT_TYPES
+    ---@param msg any
     local _send = function (msg_type, msg)
         local s_pkt = comms.scada_packet()
         local m_pkt = comms.mgmt_packet()
@@ -164,6 +192,7 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
     end
 
     -- keep alive ack
+    ---@param srv_time integer
     local _send_keep_alive_ack = function (srv_time)
         _send(SCADA_MGMT_TYPES.KEEP_ALIVE, { srv_time, util.time() })
     end
@@ -171,7 +200,8 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
     -- PUBLIC FUNCTIONS --
 
     -- send a MODBUS TCP packet
-    local send_modbus = function (m_pkt)
+    ---@param m_pkt modbus_packet
+    public.send_modbus = function (m_pkt)
         local s_pkt = comms.scada_packet()
         s_pkt.make(self.seq_num, PROTOCOLS.MODBUS_TCP, m_pkt.raw_sendable())
         self.modem.transmit(self.s_port, self.l_port, s_pkt.raw_sendable())
@@ -179,7 +209,9 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
     end
 
     -- reconnect a newly connected modem
-    local reconnect_modem = function (modem)
+    ---@param modem table
+---@diagnostic disable-next-line: redefined-local
+    public.reconnect_modem = function (modem)
         self.modem = modem
 
         -- open modem
@@ -189,42 +221,43 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
     end
 
     -- unlink from the server
-    local unlink = function (rtu_state)
+    ---@param rtu_state rtu_state
+    public.unlink = function (rtu_state)
         rtu_state.linked = false
         self.r_seq_num = nil
     end
 
     -- close the connection to the server
-    local close = function (rtu_state)
+    ---@param rtu_state rtu_state
+    public.close = function (rtu_state)
         self.conn_watchdog.cancel()
-        unlink(rtu_state)
+        public.unlink(rtu_state)
         _send(SCADA_MGMT_TYPES.CLOSE, {})
     end
 
     -- send capability advertisement
-    local send_advertisement = function (units)
+    ---@param units table
+    public.send_advertisement = function (units)
         local advertisement = {}
 
         for i = 1, #units do
-            local unit = units[i]
+            local unit = units[i]           ---@type rtu_unit_registry_entry
             local type = comms.rtu_t_to_advert_type(unit.type)
 
             if type ~= nil then
+                ---@class rtu_advertisement
+                local advert = {
+                    type = type,            ---@type integer
+                    index = unit.index,     ---@type integer
+                    reactor = unit.reactor, ---@type integer
+                    rsio = nil              ---@type table|nil
+                }
+
                 if type == RTU_ADVERT_TYPES.REDSTONE then
-                    insert(advertisement, {
-                        type = type,
-                        index = unit.index,
-                        reactor = unit.for_reactor,
-                        rsio = unit.device
-                    })
-                else
-                    insert(advertisement, {
-                        type = type,
-                        index = unit.index,
-                        reactor = unit.for_reactor,
-                        rsio = nil
-                    })
+                    advert.rsio = unit.device
                 end
+
+                insert(advertisement, advert)
             end
         end
 
@@ -232,7 +265,13 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
     end
 
     -- parse a MODBUS/SCADA packet
-    local parse_packet = function(side, sender, reply_to, message, distance)
+    ---@param side string
+    ---@param sender integer
+    ---@param reply_to integer
+    ---@param message any
+    ---@param distance integer
+    ---@return modbus_frame|mgmt_frame|nil packet
+    public.parse_packet = function(side, sender, reply_to, message, distance)
         local pkt = nil
         local s_pkt = comms.scada_packet()
 
@@ -261,10 +300,11 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
     end
 
     -- handle a MODBUS/SCADA packet
-    local handle_packet = function(packet, units, rtu_state)
+    ---@param packet modbus_frame|mgmt_frame
+    ---@param units table
+    ---@param rtu_state rtu_state
+    public.handle_packet = function(packet, units, rtu_state)
         if packet ~= nil then
-            local seq_ok = true
-
             -- check sequence number
             if self.r_seq_num == nil then
                 self.r_seq_num = packet.scada_frame.seq_num()
@@ -281,26 +321,27 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
             local protocol = packet.scada_frame.protocol()
 
             if protocol == PROTOCOLS.MODBUS_TCP then
+                local return_code = false
                 local reply = modbus.reply__neg_ack(packet)
 
                 -- handle MODBUS instruction
                 if packet.unit_id <= #units then
-                    local unit = units[packet.unit_id]
+                    local unit = units[packet.unit_id]  ---@type rtu_unit_registry_entry
                     if unit.name == "redstone_io" then
                         -- immediately execute redstone RTU requests
-                        local return_code, reply = unit.modbus_io.handle_packet(packet)
+                        return_code, reply = unit.modbus_io.handle_packet(packet)
                         if not return_code then
                             log.warning("requested MODBUS operation failed")
                         end
                     else
                         -- check validity then pass off to unit comms thread
-                        local return_code, reply = unit.modbus_io.check_request(packet)
+                        return_code, reply = unit.modbus_io.check_request(packet)
                         if return_code then
                             -- check if an operation is already in progress for this unit
                             if unit.modbus_busy then
                                 reply = unit.modbus_io.reply__srv_device_busy(packet)
                             else
-                                unit.pkt_queue.push(packet)
+                                unit.pkt_queue.push_packet(packet)
                             end
                         else
                             log.warning("cannot perform requested MODBUS operation")
@@ -312,7 +353,7 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
                     log.error("MODBUS packet requesting non-existent unit")
                 end
 
-                send_modbus(reply)
+                public.send_modbus(reply)
             elseif protocol == PROTOCOLS.SCADA_MGMT then
                 -- SCADA management packet
                 if packet.type == SCADA_MGMT_TYPES.KEEP_ALIVE then
@@ -334,7 +375,7 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
                 elseif packet.type == SCADA_MGMT_TYPES.CLOSE then
                     -- close connection
                     self.conn_watchdog.cancel()
-                    unlink(rtu_state)
+                    public.unlink(rtu_state)
                     println_ts("server connection closed by remote host")
                     log.warning("server connection closed by remote host")
                 elseif packet.type == SCADA_MGMT_TYPES.REMOTE_LINKED then
@@ -343,7 +384,7 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
                     self.r_seq_num = nil
                 elseif packet.type == SCADA_MGMT_TYPES.RTU_ADVERT then
                     -- request for capabilities again
-                    send_advertisement(units)
+                    public.send_advertisement(units)
                 else
                     -- not supported
                     log.warning("RTU got unexpected SCADA message type " .. packet.type)
@@ -355,15 +396,7 @@ rtu.comms = function (modem, local_port, server_port, conn_watchdog)
         end
     end
 
-    return {
-        send_modbus = send_modbus,
-        reconnect_modem = reconnect_modem,
-        parse_packet = parse_packet,
-        handle_packet = handle_packet,
-        send_advertisement = send_advertisement,
-        unlink = unlink,
-        close = close
-    }
+    return public
 end
 
 return rtu

rtu/startup.lua

@@ -22,7 +22,7 @@ local imatrix_rtu = require("rtu.dev.imatrix_rtu")
 local turbine_rtu = require("rtu.dev.turbine_rtu")
 local turbinev_rtu = require("rtu.dev.turbinev_rtu")
 
-local RTU_VERSION = "alpha-v0.6.2"
+local RTU_VERSION = "alpha-v0.6.3"
 
 local rtu_t = types.rtu_t
 
@@ -45,8 +45,10 @@ println(">> RTU " .. RTU_VERSION .. " <<")
 -- mount connected devices
 ppm.mount_all()
 
+---@class rtu_shared_memory
 local __shared_memory = {
     -- RTU system state flags
+    ---@class rtu_state
     rtu_state = {
         linked = false,
         shutdown = false
@@ -59,9 +61,9 @@ local __shared_memory = {
 
     -- system objects
     rtu_sys = {
-        rtu_comms = nil,
-        conn_watchdog = nil,
-        units = {}
+        rtu_comms = nil,        ---@type rtu_comms
+        conn_watchdog = nil,    ---@type watchdog
+        units = {}              ---@type table
     },
 
     -- message queues
@@ -140,7 +142,8 @@ for reactor_idx = 1, #rtu_redstone do
         end
     end
 
-    table.insert(units, {
+    ---@class rtu_unit_registry_entry
+    local unit = {
         name = "redstone_io",
         type = rtu_t.redstone,
         index = 1,
@@ -151,7 +154,9 @@ for reactor_idx = 1, #rtu_redstone do
         modbus_busy = false,
         pkt_queue = nil,
         thread = nil
-    })
+    }
+
+    table.insert(units, unit)
 
     log.debug("init> initialized RTU unit #" .. #units .. ": redstone_io (redstone) [1] for reactor " .. rtu_redstone[reactor_idx].for_reactor)
 end
@@ -201,6 +206,7 @@ for i = 1, #rtu_devices do
         end
 
         if rtu_iface ~= nil then
+            ---@class rtu_unit_registry_entry
             local rtu_unit = {
                 name = rtu_devices[i].name,
                 type = rtu_type,

rtu/threads.lua

@@ -1,11 +1,9 @@
-local comms = require("scada-common.comms")
 local log = require("scada-common.log")
 local mqueue = require("scada-common.mqueue")
 local ppm = require("scada-common.ppm")
 local types = require("scada-common.types")
 local util = require("scada-common.util")
 
-local redstone_rtu = require("rtu.dev.redstone_rtu")
 local boiler_rtu = require("rtu.dev.boiler_rtu")
 local boilerv_rtu = require("rtu.dev.boilerv_rtu")
 local energymachine_rtu = require("rtu.dev.energymachine_rtu")
@@ -24,12 +22,11 @@ local println = util.println
 local print_ts = util.print_ts
 local println_ts = util.println_ts
 
-local psleep = util.psleep
-
 local MAIN_CLOCK  = 2   -- (2Hz, 40 ticks)
 local COMMS_SLEEP = 150 -- (150ms, 3 ticks)
 
 -- main thread
+---@param smem rtu_shared_memory
 threads.thread__main = function (smem)
     -- execute thread
     local exec = function ()
@@ -42,7 +39,7 @@ threads.thread__main = function (smem)
         local rtu_state     = smem.rtu_state
         local rtu_dev       = smem.rtu_dev
         local rtu_comms     = smem.rtu_sys.rtu_comms
-        local conn_watchdog = smem.rtu_sys.conn_watchdog    ---@type watchdog
+        local conn_watchdog = smem.rtu_sys.conn_watchdog
         local units         = smem.rtu_sys.units
 
         -- start clock
@@ -116,7 +113,7 @@ threads.thread__main = function (smem)
                     else
                         -- relink lost peripheral to correct unit entry
                         for i = 1, #units do
-                            local unit = units[i]
+                            local unit = units[i]   ---@type rtu_unit_registry_entry
 
                             -- find disconnected device to reconnect
                             if unit.name == param1 then
@@ -137,7 +134,7 @@ threads.thread__main = function (smem)
                                     unit.rtu = imatrix_rtu.new(device)
                                 end
 
-                                unit.modbus_io = modbus.new(unit.rtu)
+                                unit.modbus_io = modbus.new(unit.rtu, true)
 
                                 println_ts("reconnected the " .. unit.type .. " on interface " .. unit.name)
                             end
@@ -159,6 +156,7 @@ threads.thread__main = function (smem)
 end
 
 -- communications handler thread
+---@param smem rtu_shared_memory
 threads.thread__comms = function (smem)
     -- execute thread
     local exec = function ()
@@ -179,14 +177,16 @@ threads.thread__comms = function (smem)
             while comms_queue.ready() and not rtu_state.shutdown do
                 local msg = comms_queue.pop()
 
-                if msg.qtype == mqueue.TYPE.COMMAND then
-                    -- received a command
-                elseif msg.qtype == mqueue.TYPE.DATA then
-                    -- received data
-                elseif msg.qtype == mqueue.TYPE.PACKET then
-                    -- received a packet
-                    -- handle the packet (rtu_state passed to allow setting link flag)
-                    rtu_comms.handle_packet(msg.message, units, rtu_state)
+                if msg ~= nil then
+                    if msg.qtype == mqueue.TYPE.COMMAND then
+                        -- received a command
+                    elseif msg.qtype == mqueue.TYPE.DATA then
+                        -- received data
+                    elseif msg.qtype == mqueue.TYPE.PACKET then
+                        -- received a packet
+                        -- handle the packet (rtu_state passed to allow setting link flag)
+                        rtu_comms.handle_packet(msg.message, units, rtu_state)
+                    end
                 end
 
                 -- quick yield
@@ -209,6 +209,8 @@ threads.thread__comms = function (smem)
 end
 
 -- per-unit communications handler thread
+---@param smem rtu_shared_memory
+---@param unit rtu_unit_registry_entry
 threads.thread__unit_comms = function (smem, unit)
     -- execute thread
     local exec = function ()
@@ -227,16 +229,18 @@ threads.thread__unit_comms = function (smem, unit)
             while packet_queue.ready() and not rtu_state.shutdown do
                 local msg = packet_queue.pop()
 
-                if msg.qtype == mqueue.TYPE.COMMAND then
-                    -- received a command
-                elseif msg.qtype == mqueue.TYPE.DATA then
-                    -- received data
-                elseif msg.qtype == mqueue.TYPE.PACKET then
-                    -- received a packet
-                    unit.modbus_busy = true
-                    local return_code, reply = unit.modbus_io.handle_packet(msg.message)
-                    rtu_comms.send_modbus(reply)
-                    unit.modbus_busy = false
+                if msg ~= nil then
+                    if msg.qtype == mqueue.TYPE.COMMAND then
+                        -- received a command
+                    elseif msg.qtype == mqueue.TYPE.DATA then
+                        -- received data
+                    elseif msg.qtype == mqueue.TYPE.PACKET then
+                        -- received a packet
+                        unit.modbus_busy = true
+                        local _, reply = unit.modbus_io.handle_packet(msg.message)
+                        rtu_comms.send_modbus(reply)
+                        unit.modbus_busy = false
+                    end
                 end
 
                 -- quick yield