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Modbus
Modbus is an open, serial
communuciations protocol based on
a master-slave architecture. Since it is
very easy to implement on any kind
of serial interface, it can be used in a
wide range of applications. Modbus
comprises a master and several
slaves, whereby comunication is
controlled exclusively by the master.
Modbus features two basic
communication mechanisms:
• Question/answer (polling): The
master sends an inquiry to a
station and waits for a response.
• Broadcast: The master sends a
command to all the network
stations, which execute the
command without confirmation.
The messages enable process data
(input/output data) to be written
and read from the slaves either
individually or in groups.
The data can either be transmitted
in ASCII or as a package in RTU
format. Modbus can be used over a
wide range of transmission media,
normally on an RS485 physical bus,
a twisted, shielded two-wire cable
with terminating resistors (same for
the PROFIBUS-DP).
The Modbus protocol was originally
developed for networking control
systems, and is often used for
connecting input/output modules
to a central PLC. Due to the low
transmission rate of 38.4 kBaud
max., Modbus is recommended for
applications with a low number of
stations or low response time
requirements.
WL PROFIBUS Communication and Electronic Accessories • July 2004
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