Operating Instructions

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Siemens Engineering Guide SOC7-128 Modbus interface SE1Z5996en7
Building Technologies 2 General 2015-08-26
2 General
2.1 The Modbus protocol
The following section provides only a brief overview of the Modbus protocol.
For the full specification, refer to "Modicon Modbus Protocol Reference Guide
PI-MBUS-300 Rev. J".
The Modbus is a master/slave protocol. By definition, this means that a Modbus
network contains one, and only one, master and at least one slave.
The Modbus master starts the transactions on the network with a slave query. The
slave either responds positively with the requested service (response) or transmits an
"exception message". In the remainder of this document, these query/response
sequences are also referred to as "Modbus telegrams".
The type of transaction is defined by the function code transmitted in the Modbus
telegrams. A function code defines the following:
 Structure of the telegram, query and response
 Direction of data transmission (master
 slave or slavemaster)
 Data format of data point (bit or 16-bit register)
The Modbus protocol defines two alternative serial transmission modes:
These modes have the following characteristics:
RTU (Remote Terminal Unit) mode
 Binary-coded data
 Start and end of telegrams marked by timed pauses (a "silent interval") between
the characters transmitted.
 Check sum algorithm: CRC (Cyclic Redundancy Check)
ASCII mode
 Data in hexadecimal notation
 Beginning and end of telegrams marked by start and end characters.
 Check sum algorithm: LRC (Longitudinal Redundancy Check)
Certain types of Modbus transactions permit the transmission of a variable number of
Modbus data points (bit or 16-bit register) in a single telegram.
A Modbus TCP/RTU gateway can be used to connect a Modbus/TCP master to one or
several slaves. The Modbus TCP/RTU gateway will act as a Modbus/TCP slave on a
Ethernet network, and transform the queries to the serial Modbus network and back
again. See also chapter 3.3 for hints when communication via Ethernet to SOC7-128.
Master/slave protocol
Transactions on the
Modbus
Function codes
Transmission modes
Telegrams with
multiple data points
Modbus TCP
Ethernet