User Manual

Easy to communicate between devices
7.10 PtP, USS, and Modbus communication protocols
Easy Book
Manual, 03/2014, A5E02486774-AF
167
7.10.5
Modbus instructions
The CPU supports Modbus communication over different networks:
● Modbus RTU (Remote Terminal Unit) is a standard network communication protocol that
uses the RS232 or RS485 electrical connection for serial data transfer between Modbus
network devices. You can add PtP (Point to Point) network ports to a CPU with a RS232
or RS485 CM or a RS485 CB.
Modbus RTU uses a master/slave network where all communications are initiated by a
single Master device and slaves can only respond to a master’s request. The master
sends a request to one slave address and only that slave address responds to the
command.
● Modbus TCP (Transmission Control Protocol) is a standard network communication
protocol that uses the PROFINET connector on the CPU for TCP/IP communication. No
additional communication hardware module is required.
Modbus TCP uses client-server connections as a Modbus communication path. Multiple
client-server connections may exist, in addition to the connection between STEP 7 and
the CPU. Mixed client and server connections are supported up to the maximum number
of connections allowed by the CPU. Each MB_SERVER connection must use a unique
instance DB and IP port number. Only 1 connection per IP port is supported. Each
MB_SERVER (with its unique instance DB and IP port) must be executed individually for
each connection.
WARNING
If an attacker can physically access your networks, the attacker can possibly read and
write data.
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 blocks.
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, please see our "Operational Guidelines
for Industrial Security" on the Service and Support site:
www.industry.siemens.com/topics/global/en/industrial-
security/Documents/operational_guidelines_industrial_security_en.pdf
(http://www.industry.siemens.com/topics/global/en/industrial-
security/Documents/operational_guidelines_industrial_security_en.pdf)
Note
Modbus
TCP will only operate correctly with CPU firmware release V1.02 or later. An
attempt to execute the Modbus instructions on an earlier firmware version will result in an
error.