Dräger Polytron 8000 Series Technical Manual page 7

2.2.2 Transmission technology
RS-485
RS-485 is the most common transmission technology. It uses a shielded, twisted 2-
wire line. Transmission rates between 9600 bit/s and 115200 bit/s can be selected.
It is defined when commissioning the system and is the same for all devices
connected to the bus. Up to 32 bus participants can be connected per segment.
The maximum permissible line length depends on the transmission rate.
Several different network topologies are possible:
– Backbone
– Star
– Ring
– Daisy chain
According to experience, a RS-485 network works best in a daisy chain topology. In
this topology, reflections on the network line are absorbed most effectively.
Serial RS-485 connection - not connected correctly
Serial RS-485 connection - connected correctly
Characteristics of the bus connection
The most important characteristics of the bus connection with their physical and
electrical properties are:
Network topology
Transmission medium
Line lengths (without repeaters) Maximum line length: 1000 m
Physical interface
Addressing
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Technical Manual
Digital Communication - Modbus RTU
Device 1 Device 2
COM
+ -
Device 1
Device 2
COM
+ -
Basic principles of the Modbus technology
Device 3
COM
+ -
Device 3
COM
+ -
Linear bus; if required, with bus termination at
both ends
Shielded, twisted 2-wire copper cable
RS-485 with bus connector in the top hat rail,
can be installed on site
1 to 247
(32 devices in one segment. Can be extended
to 247 with repeater)
COM
+ -
COM
+ -
7