Correct the calculation of the PDU size in bytes. Rather than determined empirically, used https://modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf page 32.

Author: theobarkerh

pymodbus/file_message.py

@@ -140,13 +140,16 @@ def execute(self, context):  # NOSONAR pylint: disable=unused-argument,no-self-u
 
     def get_response_pdu_size(self):
         """
-        Func_code (1 byte) + Byte Count(1 byte) + Byte Count2 (1 byte) + Reference Type (1 byte) + registers to read for file
+        Func_code (1 byte) + Response Byte Count(1 byte) +
+            N * (File Response Byte Count (1 byte) + Reference Type (1 byte) + 2 * registers to read for file record)
+            where N = record sub-request count.
         :return: 
         """
+        self.count = 0
         for record in self.records:
-            self.count+=record.record_length
+            self.count+= record.record_length * 2 + 2 if record.record_length else 0
 
-        return 4 + self.count
+        return 2 + self.count
 
 class ReadFileRecordResponse(ModbusResponse):
     """Read file record response.