Bus Cable - Drehmo i-matic Operating Manual

3.6 Bus cable

Only cables according to the specification for RS-485 (MODBUS RTU) should be used:
Impedance
Line capacity
Wire diameter
Wire gauge
Loop impedance
Shielding
Respect a distance of minimum 20 cm between the bus cable and other cables. If possible, bus
cables should be laid in a separate, conductive, and earthed cable tray.
Make sure to avoid potential differences between the individual devices on the bus (perform a
potential equalization).
There is not standardized color marking for RS485 wiring.
Usually the marking is a follows: P/B = brown and N/A = white
The maximum values at a given transmission rate in conjunction with the above given cable
parameters are as follows:
Bit rate [kbit/s] Bit time
300 3.3 ms
600 1.7 ms
1200 833.3 µs
2400 416.7 µs
4800 208.3 µs
9600 104 µs
19200 52 µs
38400 26 µs
57600 17.36 µs
115200 8,68 µs
1
See reference: ‚Modbus over serial line'
DREHMO – i-matic Modbus loop redundant board
Complementary operating manual
135 to 165 Ohm, at a test frequency of 3 to 20 MHz
< 30 pF per meter
> 0,64 mm
> 0,34 mm² (equivalent AWG 22), max. 1,5 mm²
< 110 Ohm per km
Copper mesh screen or mesh screen with foil screen
Max. segment length [m]
1200
1200
1200
1200
1200
1200
1200
1200
1200
1000
Max. stub length [m]
500
500
500
500
500
500
500
100
100
33
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