diff --git a/CHANGELOG.md b/CHANGELOG.md
index f20a43b..22b3a83 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+# [3.1.1] - 2023-01-25
+
+* Fixed `Packet::isCompleteLengthRTU()` to differentiate fix sized function code responses from variable sized responses.
+
 # [3.1.0] - 2022-10-16
 
 * Added `Packet::isCompleteLengthRTU()` to help checking if packet is complete RTU packet. Helps when receiving fragmented packets.
diff --git a/src/Utils/Packet.php b/src/Utils/Packet.php
index b7f2ce5..3493388 100644
--- a/src/Utils/Packet.php
+++ b/src/Utils/Packet.php
@@ -6,6 +6,7 @@
 
 use ModbusTcpClient\Network\IOException;
 use ModbusTcpClient\Packet\ErrorResponse;
+use ModbusTcpClient\Packet\ModbusPacket;
 
 final class Packet
 {
@@ -39,8 +40,8 @@ public static function isCompleteLength(string|null $binaryData): bool
     }
 
     /**
-     * isCompleteLengthRTU checks if binary string is complete modbus RTU packet
-     * NB: this function works only for MODBUS RTU packets
+     * isCompleteLengthRTU checks if binary string is complete modbus RTU response packet
+     * NB: this function works only for MODBUS RTU response packets
      *
      * @param string|null $binaryData binary string to be checked
      * @return bool true if data is actual error packet
@@ -52,17 +53,36 @@ public static function isCompleteLengthRTU(string|null $binaryData): bool
         if ($length < 5) {
             return false;
         }
-        if ((ord($binaryData[1]) & ErrorResponse::EXCEPTION_BITMASK) > 0) { // seems to be error response
+        $functionCode = ord($binaryData[1]);
+        if (($functionCode & ErrorResponse::EXCEPTION_BITMASK) > 0) { // seems to be error response
             return true;
         }
-        // if it is not error response then 3rd byte contains data length in bytes
+        switch ($functionCode) {
+            case ModbusPacket::READ_COILS: //
+            case ModbusPacket::READ_INPUT_DISCRETES: //
+            case ModbusPacket::READ_HOLDING_REGISTERS: //
+            case ModbusPacket::READ_INPUT_REGISTERS: //
+            case ModbusPacket::READ_WRITE_MULTIPLE_REGISTERS: //
+                // if it is not error response then 3rd byte contains data length in bytes
 
-        // trailing 3 bytes are = unit id + function code + data length in bytes
-        // next is N bytes of data that should match 3rd byte value
-        // and 2 bytes for CRC
-        // so adding these number is what complete packet would be
-        $expectedLength = 3 + ord($binaryData[2]) + 2;
+                // trailing 3 bytes are = unit id (1) + function code (1) + data length in bytes (1) + (N)
+                // next is N bytes of data that should match 3rd byte value
+                $responseBytesLen = 3 + ord($binaryData[2]);
+                break;
+            case ModbusPacket::WRITE_SINGLE_COIL: // unit id (1) + function code (1) + start address (2) + coil data (2)
+            case ModbusPacket::WRITE_SINGLE_REGISTER: // unit id (1) + function code (1) + start address (2) + register data (2)
+            case ModbusPacket::WRITE_MULTIPLE_COILS: // unit id (1) + function code (1) + start address (2) + count of coils written (2)
+            case ModbusPacket::WRITE_MULTIPLE_REGISTERS: // unit id (1) + function code (1) + start address (2) + count of registers written (2)
+                $responseBytesLen = 6;
+                break;
+            case ModbusPacket::MASK_WRITE_REGISTER:
+                $responseBytesLen = 8; // unit id (1) + function code (1) + start address (2) + AND mask (2) + OR mask (2)
+                break;
+            default:
+                throw new IOException('can not determine complete length for unsupported modbus function code');
+        }
 
+        $expectedLength = $responseBytesLen + 2; // and 2 bytes for CRC
         if ($length > $expectedLength) {
             throw new IOException('packet length more bytes than expected');
         }