RS485 cable type and speed Section 2 Transmission Technology
26 3BDS009029R5001 B
transmission rates 31.25 and 45.45 kbit/s. The default value is 1.5 Mbit/s. Higher
values require careful consideration of correct cable installation. The PROFIBUS
configuration tool within the 800xA system is the Control Builder M. This tool
calculates the bus timing parameter and checks the configuration values, inserted by
the user. For example a compilation warning occurs if a slave does not support the
selected transmission rate. In this case the next lower supported transmission rate
will be selected automatically. Changing the transmission rate may also require
changing of other bus parameters to ensure successful communication. The
automatic calculation will determine the right settings. The adjusted speed is valid
for all connected devices on the bus, when commissioning the system.
RS485 cable type and speed
The maximum permissible bus length depends on the transmission rate and the bus
cable type. Different cable types (designation A to D) for different applications are
available for connecting PROFIBUS devices either each other or to network
elements (Linking Devices, Repeaters).
The electrical lines are shielded twisted two-wire cables with a circular cross-
section. The RS 485 interface operates on voltage differences. It is therefore less
sensitive to interference than a voltage or current interface. Depending on the
transmission speed, segments can be implemented in lengths of from 100 meter to
over 1000 meter. The bus cable to be used for the bus modules is specified in DIN
19245/EN 50170 as cable type A and can be used according to the following table:
A detailed description of the AC 800M and CI854(A) is available in their specific
product manuals.
Table 2. Basic data for a PROFIBUS DP reference cable
Parameter DP, Cable type A
Cable design Twisted pair and shielded
Surge impedance 135 ... 165 Ohm at f=3 ... 20 MHz
Operating capacity =< 30 pF/m
Loop resistance =< 110 Ohm/km
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