Modbus Client and Server Roles

Modbus Client and Server Gateway Responsibilities

RAPTOR Client and server gateways do the following:

• Translate TCP messages to serial messages and vise versa.
• Act like a pipe encapsulating serial modbus messages in TCP packets.
• Do not check the validity of function codes and data inside messages. This is done in master and slave applications.
• Create exceptions and send them to the proper destination.

The RAPTOR should be used as a client or a server when the Master and Slave are not native TCP speakers.

Modbus Server Role

The RAPTOR when used as a modbus server can do the following:

• Accept up to 64 connections from clients
• Work with multiple serial interfaces simultaneously
• Support up to 247 slaves on all serial interfaces. Please note that a much lower number of slaves is recommended for reasons surrounding serial link contention.
• Receive TCP modbus requests from TCP modbus clients
• Validates received messages for length, function code and slave ID
• Returns proper exception in the case of validation errors and if return exceptions are enabled
• If there are no active transactions on the interface:
  • Converts the TCP modbus message to a serial modbus message
  • Sends serial messages to the proper serial interface
  • Keeps the original message for response validation
  • Starts the response timer
• If ther is an active transaction on the interface, stores the request in a queue that can hold up to 16 requests.
• When the active transaction is over (by receiving a response or no response in the configured time), initiates new transaction by pulling the next available request from the requests queue.
• Validates received response (length, CRC, function code, slave ID) against the stored original request.
• Converts received responses to TCP messages.
• Sends the TCP messages to the proper master.

Modbus Client Role

The RAPTOR when used as a modbus client can do the following:

• Receives serial commands from the serial master
• Validates received messages (length, CRC, function code, slave ID)
• Returns the proper exception in the case of validation error and if return exceptions are enabled.
• Converts the serial modbus message to a TCP modbus message.
• Sends the TCP message to the server while keeping the original request for the response validation.
• Receives a response, and validates it using the original request.
• Converts received TCP messages to serial messages.
• Sends converted serial messages to the serial master.
• Starts the response timer.
• In case of no response from the server, sends proper exception to the master.

Modbus Function and Exception Codes

A response function code is valid if:

• The response function code matches the request function code.
• The response function code is an exception created using the request function code.

The exception function code is equivalent to the request function code with its most significant bit set to logic 1. Example: the 0x83 code is the exception for the function code 0x03.