Modbus Communication; Overview Of Modbus Rtu And Modbus Tcp Communication - Siemens SIMATIC S7-1200 System Manual

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Communication processor and Modbus TCP

13.5 Modbus communication

Modbus communication

Modbus function codes

● A CPU operating as a Modbus RTU master (or Modbus TCP client) can read/write both

data and I/O states in a remote Modbus RTU slave (or Modbus TCP server). Remote

data can be read and then processed in your program logic.

● A CPU operating as a Modbus RTU slave (or Modbus TCP server) allows a supervisory

device to read/write both data and I/O states in CPU memory. An RTU master (or

Modbus TCP client) can write new values into slave/server CPU memory that is available

to your program logic.

If an attacker can physically access your networks, the attacker can possibly read and write

The TIA Portal, the CPU, and HMIs (except HMIs using GET/PUT) use secure

communication that protects against replay and "man-in-the-middle" attacks. Once

communication is enabled, the exchange of signed messages takes place in clear text

which allows an attacker to read data, but protects against unauthorized writing of data.

The TIA Portal, not the communication process, encrypts the data of know-how protected

All other forms of communication (I/O exchange through PROFIBUS, PROFINET, AS-i, or

other I/O bus, GET/PUT, T-Block, and communication modules (CM)) have no security

features. You must protect these forms of communication by limiting physical access. If an

attacker can physically access your networks utilizing these forms of communication, the

attacker can possibly read and write data.

For security information and recommendations, refer to the "Operational Guidelines for

Industrial Security"

security/Documents/operational_guidelines_industrial_security_en.pdf).

Table 13- 52 Read data functions: Read remote I/O and program data

Modbus function code

(http://www.industry.siemens.com/topics/global/en/industrial-

Read slave (server) functions - standard addressing

Read output bits: 1 to 2000 bits per request

Read input bits: 1 to 2000 bits per request

Read Holding registers: 1 to 125 words per request

Read input words: 1 to 125 words per request

S7-1200 Programmable controller

System Manual, V4.2, 09/2016, A5E02486680-AK