From 912011bfede5c239cbf170a203a1a9d575d715f8 Mon Sep 17 00:00:00 2001
From: Mikayla Fischler <mikayla@ky8.io>
Date: Fri, 22 Apr 2022 20:21:28 -0400
Subject: [PATCH] #30 modbus comms changes

---
 scada-common/comms.lua  | 17 ++++++--------
 scada-common/modbus.lua | 51 ++++++++++++++++++++++++++++-------------
 2 files changed, 42 insertions(+), 26 deletions(-)

diff --git a/scada-common/comms.lua b/scada-common/comms.lua
index c4b8791..972f768 100644
--- a/scada-common/comms.lua
+++ b/scada-common/comms.lua
@@ -88,7 +88,7 @@ function scada_packet()
     -- public accessors --
 
     local modem_event = function () return self.modem_msg_in end
-    local raw = function () return self.raw end
+    local raw_sendable = function () return self.raw end
 
     local sender = function () return self.s_port end
     local receiver = function () return self.r_port end
@@ -106,7 +106,7 @@ function scada_packet()
         receive = receive,
         -- raw access
         modem_event = modem_event,
-        raw = raw,
+        raw_sendable = raw_sendable,
         -- sender/receiver
         sender = sender,
         receiver = receiver,
@@ -121,13 +121,12 @@ function scada_packet()
 end
 
 -- MODBUS packet 
--- modeled after MODBUS TCP packet, but length is not transmitted since these are tables (#data = length, easy)
+-- modeled after MODBUS TCP packet
 function modbus_packet()
     local self = {
         frame = nil,
         raw = nil,
         txn_id = txn_id,
-        protocol = protocol,
         length = length,
         unit_id = unit_id,
         func_code = func_code,
@@ -135,16 +134,15 @@ function modbus_packet()
     }
 
     -- make a MODBUS packet
-    local make = function (txn_id, protocol, unit_id, func_code, data)
+    local make = function (txn_id, unit_id, func_code, data)
         self.txn_id = txn_id
-        self.protocol = protocol
         self.length = #data
         self.unit_id = unit_id
         self.func_code = func_code
         self.data = data
 
         -- populate raw array
-        self.raw = { self.txn_id, self.protocol, self.unit_id, self.func_code }
+        self.raw = { self.txn_id, self.unit_id, self.func_code }
         for i = 1, self.length do
             table.insert(self.raw, data[i])
         end
@@ -156,11 +154,11 @@ function modbus_packet()
             self.frame = frame
 
             if frame.protocol() == PROTOCOLS.MODBUS_TCP then
-                local size_ok = frame.length() >= 4
+                local size_ok = frame.length() >= 3
     
                 if size_ok then
                     local data = frame.data()
-                    make(data[1], data[2], data[3], data[4], { table.unpack(data, 5, #data) })
+                    make(data[1], data[2], data[3], { table.unpack(data, 4, #data) })
                 end
     
                 return size_ok
@@ -182,7 +180,6 @@ function modbus_packet()
         return {
             scada_frame = self.frame,
             txn_id = self.txn_id,
-            protocol = self.protocol,
             length = self.length,
             unit_id = self.unit_id,
             func_code = self.func_code,
diff --git a/scada-common/modbus.lua b/scada-common/modbus.lua
index a8148b7..494fde8 100644
--- a/scada-common/modbus.lua
+++ b/scada-common/modbus.lua
@@ -203,10 +203,10 @@ function new(rtu_dev)
         return return_ok, response
     end
 
+    -- handle a MODBUS TCP packet and generate a reply
     local handle_packet = function (packet)
         local return_code = true
         local response = nil
-        local reply = packet
 
         if #packet.data == 2 then
             -- handle  by function code
@@ -236,32 +236,51 @@ function new(rtu_dev)
             return_code = false
         end
 
-        if return_code then
-            -- default is to echo back
-            if type(response) == "table" then
-                reply.length = #response
-                reply.data = response
-            end
-        else
+        -- default is to echo back
+        local func_code = packet.func_code
+        if not return_code then
             -- echo back with error flag
-            reply.func_code = bit.bor(packet.func_code, MODBUS_FCODE.ERROR_FLAG)
+            func_code = bit.bor(packet.func_code, MODBUS_FCODE.ERROR_FLAG)
 
             if type(response) == "nil" then
-                reply.length = 0
-                reply.data = {}
+                response = { }
             elseif type(response) == "number" then
-                reply.length = 1
-                reply.data = { response }
+                response = { response }
             elseif type(response) == "table" then
-                reply.length = #response
-                reply.data = response
+                response = response
             end
         end
 
+        -- create reply
+        local reply = comms.modbus_packet()
+        reply.make(packet.txn_id, packet.unit_id, func_code, response)
+
         return return_code, reply
     end
 
+    -- return a NEG_ACKNOWLEDGE error reply
+    local 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)
+        local data = { MODBUS_EXCODE.NEG_ACKNOWLEDGE }
+        reply.make(packet.txn_id, packet.unit_id, fcode, data)
+        return reply
+    end
+
+    -- return a GATEWAY_PATH_UNAVAILABLE error reply
+    local 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)
+        local data = { MODBUS_EXCODE.GATEWAY_PATH_UNAVAILABLE }
+        reply.make(packet.txn_id, packet.unit_id, fcode, data)
+        return reply
+    end
+
     return {
-        handle_packet = handle_packet
+        handle_packet = handle_packet,
+        reply__neg_ack = reply__neg_ack,
+        reply__gw_unavailable = reply__gw_unavailable
     }
 end