From 58c3333fcc85a1686594306c97e697998015d339 Mon Sep 17 00:00:00 2001
From: Stefan Allius <stefan.allius@t-online.de>
Date: Thu, 2 May 2024 23:55:59 +0200
Subject: [PATCH] initial checkin

---
 app/src/modbus.py        | 56 ++++++++++++++++++++++++++++++++++++++++
 app/src/singleton.py     |  9 +++++++
 app/tests/test_modbus.py | 21 +++++++++++++++
 3 files changed, 86 insertions(+)
 create mode 100644 app/src/modbus.py
 create mode 100644 app/src/singleton.py
 create mode 100644 app/tests/test_modbus.py

diff --git a/app/src/modbus.py b/app/src/modbus.py
new file mode 100644
index 0000000..9745f23
--- /dev/null
+++ b/app/src/modbus.py
@@ -0,0 +1,56 @@
+import struct
+
+if __name__ == "app.src.modbus":
+    from app.src.singleton import Singleton
+else:  # pragma: no cover
+    from singleton import Singleton
+
+#######
+# TSUN uses the Modbus in the RTU transmission mode.
+# see: https://modbus.org/docs/Modbus_over_serial_line_V1_02.pdf
+#
+# A Modbus PDU consists of: 'Function-Code' + 'Data'
+# A Modbus RTU message consists of: 'Addr' + 'Modbus-PDU' + 'CRC-16'
+#
+# The 16-bit CRC is known as CRC-16-ANSI(reverse)
+# see: https://en.wikipedia.org/wiki/Computation_of_cyclic_redundancy_checks
+#######
+
+CRC_POLY = 0xA001  # (LSBF/reverse)
+CRC_INIT = 0xFFFF
+
+
+class Modbus(metaclass=Singleton):
+    MB_WRITE_SINGLE_REG = 6
+    MB_READ_SINGLE_REG = 3
+    __crc_tab = []
+
+    def __init__(self):
+        self.__build_crc_tab(CRC_POLY)
+
+    def build_msg(self, addr, func, reg, val):
+        msg = struct.pack('>BBHH', addr, func, reg, val)
+        msg += struct.pack('<H', self.__calc_crc(msg))
+        return msg
+
+    def check_crc(self, msg) -> bool:
+        return 0 == self.__calc_crc(msg)
+
+    def __calc_crc(self, buffer: bytes) -> int:
+        crc = CRC_INIT
+
+        for cur in buffer:
+            crc = (crc >> 8) ^ self.__crc_tab[(crc ^ cur) & 0xFF]
+        return crc
+
+    def __build_crc_tab(self, poly) -> None:
+        for index in range(256):
+            data = index << 1
+            crc = 0
+            for _ in range(8, 0, -1):
+                data >>= 1
+                if (data ^ crc) & 1:
+                    crc = (crc >> 1) ^ poly
+                else:
+                    crc >>= 1
+            self.__crc_tab.append(crc)
diff --git a/app/src/singleton.py b/app/src/singleton.py
new file mode 100644
index 0000000..48778b9
--- /dev/null
+++ b/app/src/singleton.py
@@ -0,0 +1,9 @@
+class Singleton(type):
+    _instances = {}
+
+    def __call__(cls, *args, **kwargs):
+        # logger_mqtt.debug('singleton: __call__')
+        if cls not in cls._instances:
+            cls._instances[cls] = super(Singleton,
+                                        cls).__call__(*args, **kwargs)
+        return cls._instances[cls]
diff --git a/app/tests/test_modbus.py b/app/tests/test_modbus.py
new file mode 100644
index 0000000..0e9cf5b
--- /dev/null
+++ b/app/tests/test_modbus.py
@@ -0,0 +1,21 @@
+# test_with_pytest.py
+# import pytest, logging
+from app.src.modbus import Modbus
+
+
+def test_modbus_crc():
+    mb = Modbus()
+    assert 0x0b02 == mb._Modbus__calc_crc(b'\x01\x06\x20\x08\x00\x04')
+    assert 0 == mb._Modbus__calc_crc(b'\x01\x06\x20\x08\x00\x04\x02\x0b')
+    assert mb.check_crc(b'\x01\x06\x20\x08\x00\x04\x02\x0b')
+
+    assert 0xc803 == mb._Modbus__calc_crc(b'\x01\x06\x20\x08\x00\x00')
+    assert 0 == mb._Modbus__calc_crc(b'\x01\x06\x20\x08\x00\x00\x03\xc8')
+    assert mb.check_crc(b'\x01\x06\x20\x08\x00\x00\x03\xc8')
+
+def test_build_modbus_pdu():
+    mb = Modbus()
+    pdu = mb.build_msg(1,6,0x2000,0x12)
+    assert pdu == b'\x01\x06\x20\x00\x00\x12\x02\x07'
+    assert mb.check_crc(pdu)
+