Dräger Polytron 8000 Series Technical Manual page 9

Application layer – layer 7
The application layer provides functions for the application. It forms the connection
to the lower layers. Software for communication within the scope of the specific
application (measuring system, etc.) is provided by the manufacturers. In addition,
there are a number of terminal programs to enable communication between the
participants in a Modbus network. The terminal programs (e.g., Modbus Poll)
perform all functions of layers 5 to 7 for the user.
Physical layer – layer 1
The physical layer is the lowest layer and provides mechanical, electrical, and other
functional tools for establishing connections. These connections are activated,
deactivated, and used to transmit bits. These procedures serve to transmit, for
example, electrical signals, optical signals, electromagnetic waves, or sound.
Among others, the following devices and network components are assigned to the
physical layer:
– Antenna
– Repeaters
– Connector
– Socket for the network cable
– Hub
– Transceiver
– T-piece
– Terminator
Data transmission via Modbus RTU is digital, purely electrical, based on voltage
levels. Normally, 9-pin connectors and sockets are used. The pin assignment is not
standardized. Therefore always observe the manufacturer's documentation when
using different devices.
Parameter
Number of receivers
Maximum line length
Common-mode input voltage
Input resistance of the receiver
Input sensitivity of the receiver
Differential data transmission
Transmission rates
The transmission rate defines the speed at which data is transmitted. The
transmission parameters have to be the same for all devices connected to a bus.
|
Technical Manual
Digital Communication - Modbus RTU
Basic principles of the Modbus technology
RS-485/EIA485
32
1200 m
-7 to +12 V
12 kOhm
+/- 200 mV
Yes
9