2 Nomenclature and Installation
2-10
CJ-series PROFIBUS Slave Unit Operation Manual for NJ-series CPU Unit (W510)
Note 1
If network extension beyond the range of the advised cable length is required, the use of
fibre optic cable to cross the larger distance should be considered.
2
The recommended minimum cable length is 1 m.
Stub Lines
Passive Stub lines (branches from the main line) should be avoided for data transmission speeds of
more than 500 kbps. Except at end devices with termination, it is recommended to always use plug
connectors that permit two data cables to be connected directly to the plug. This method allows the
bus connector to be plugged and unplugged at all times without interrupting data communication
between other devices.
Termination Resistors
In order to minimize cable reflections and ensure a defined signal level on
the data lines, the data transfer cable must be terminated at both ends
with a terminating resistor combination. The bus termination diagram is
shown on the left.
The bus terminator connects the two data lines via a 220
Ω
resistor which,
in turn, is connected to VP 5 VDC and DGND via two 390
Ω
resistors.
Powering the terminator resistor via VP 5 VDC and DGND ensures a
defined idle state potential on the data lines.
To ensure the correct functioning up to the highest baud rate, the bus
cable must be terminated at both its ends.
A missing bus termination can cause errors during data transfer. Problems
can also arise if too many bus terminators are fitted, since each bus termi-
nator represents an electrical load and reduces the signal levels and thus
the signal-to-noise ratio. Too many or missing bus terminators can also
cause intermittent data transfer errors, particularly if the bus segment is
operated close to the specified limits for maximum numbers of devices,
maximum bus segment length and maximum data transfer rate.
In addition to the bus termination, additional precautions must be taken to
ensure proper operation at high baud rates, i.e. baud rates of 500 kbps and higher. Due to the capac-
itive load of the device and the resulting cable reflections, bus connectors must be provided with
built-in series inductors, of 110 mH each, as shown in the figure on the left.
Baud Rate
(kbps)
Distance/segment
(m)
9.6
1200
19.2
1200
45.45
1200
93.75
1200
187.5
1000
500
400
1500
200
3000
100
6000
100
12000
100
2-3-2
Bus Termination
390 Ω
390 Ω
220 Ω
VP
B-Line
A-Line
DGND
Network
B-Line
A-Line
110 mH
110 mH
Network
