Bus Termination - Omron SYSMAC CS Series Operation Manual

Setting up a Network
Cable Type
Maximum PROFIBUS
Cable Length
Note
Stub Lines
2-4-2

Bus Termination

Termination Resistors
VP
390 Ohm
B-Line
220 Ohm Network
A-Line
390 Ohm
DGND
The PROFIBUS standard EN 50170 specifies Type A shielded, twisted- pair
cable as the recommended cable type for use in an RS-485 based PROFI-
BUS network. This cable type has the following characteristics:
Characteristic
Impedance
Capacitance per unit length
Loop resistance
Core diameter
Core cross section
Note The PROFIBUS standard EN 50170 also specifies a Type B cable with differ-
ent cable characteristics. Use of Type B cable is no longer recommended.
The transmission speed defines the maximum advised cable distance or
cable segment in metres before the use of a repeater is recommended. The
cable lengths specified in the following table are based on PROFIBUS type A
cable.
Baud rate
(kbit/s)
9.6
19.2
45.45
93.75
187.5
1. If network extension beyond the range of the advised cable length is re-
quired, the use of fibre optic cable to cross the larger distance should be
considered.
2. The recommended minimum cable length is 1 m.
Passive Stub lines (branches from the main line) should be avoided for data
transmission speeds of more than 500 kbit/s. Except at end devices with ter-
mination, 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.
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 ter-
minating resistor combination. The bus termination diagram is shown on the
left.
The bus terminator connects the two data lines via a 220 ohm resistor which,
in turn, is connected to VP 5Vdc and DGND via two 390 ohm resistors. Pow-
ering the terminator resistor via VP 5V 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 termina-
tor 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
Distance/segment Baud rate
(m)
1200
1200
1200
1200
1000
Section 2-4
Value
135 - 165 ohms
< 30 pF/m
110 ohms/km
0.64 mm
2
0.34 mm
Distance/segment
(kbit/s) (m)
500 400
1500 200
3000 100
6000 100
12000 100
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