Dräger Polytron 8000 Series Technical Manual page 10

Basic principles of the Modbus technology
The following transmission rates are available for RS-485 applications:
1200 bit/s
2400 bit/s
4800 bit/s
9600 bit/s
19200 bit/s
38400 bit/s
56000 bit/s
115200 bit/s
Operating principle
Communication in a Modbus network follows a simple request/response
mechanism: the Master (1) sends a request (Telegram) (2) to the Slave (4). The
function code contained in the telegram tells the Slave which function to execute.
The Modbus application protocol defines the format in which the request initiated by
the Master is sent.
1
2
3
4
There are 3 different communication cases:
The device (Slave) receives a
valid telegram with a valid
request.
The device (Slave) receives an
invalid telegram (e.g., CRC
error).
The device (Slave) receives a
valid telegram with an invalid
request.
Communication
If a cable pair is used, transmission takes place in half-duplex mode: data can flow
alternately in both directions, but not simultaneously.
If two cable pairs are used, full-duplex mode is also possible: data is transmitted in
both directions at the same time.
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1 2
Master
Request from Master to Slave
Slave response
Slave
If it can answer this, it sends a response telegram
with the function code of this request.
It does not answer. The application (Master) has
to correct the error within a defined time (timeout).
It answers with a Modbus exception code (Excep-
tion). The application (Master) has to correct the
error.
Technical Manual
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Digital Communication - Modbus RTU
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