Signal Regeneration; Monitoring The Received Optical Level; Constant Light Monitoring - Siemens SIMATIC NET PROFIBUS Owner's Manual

Infrared Link Modul (ILM)
5.3

Signal Regeneration

The PROFIBUS ILM regenerates the signal shape and amplitude of the received signals. This makes it
possible to cascade unconnected network segments using infrared transmission links. Since the PROFIBUS
ILM, however, has a delay time for processing and passing on the signal the delay on the PROFIBUS ILM must
be taken into account. If fast response times are required in a DP system, cascading must be restricted
depending on the remaining network length and other active components in the network (repeaters, OLMs).
5.4

Monitoring the Received Optical Level

The PROFIBUS ILM monitors the received level when receiving data via the infrared link. The receive level is
compared with a fixed reference value. If the level falls below this reference value during reception, the red
"LOW" LED is always lit. With suitable configuration, the user can also trigger the signaling contact in this
situation.
The reference value corresponds to 1.4 x the minimum receive level. This situation (1.4 x the minimum receive
level) applies when the distance between the sending and receiving PROFIBUS ILM is 80 to 85% of the
maximum distance in this direction.
In the optical axis (receiver and sender are directly opposite and turned through exactly 180 degrees to each
other) the maximum distance is 15 m, in other words at approximately 12 m to 13 m between the sending and
receiving PROFIBUS ILMs, the received optical level is still 1.4 times the minimum receive level. A reserve of 2
to 3 m remains along the optical axis. This reserve is however drastically reduced if the position of one of the
PROFIBUS ILMs is changed in such a way that it is moved out of the optical axis (if the PROFIBUS ILM is
moved sideways or turned). When close together, a displacement of only a few centimeters vertically away
from the optical axis can lead not only to the level monitoring responding but also to errors on the bus.
5.5

Constant Light Monitoring

To transmit data, the PROFIBUS ILM uses infrared light as emitted by other energy sources. If the received
light exceeds a certain intensity, the working range of the receiving diode is exceeded and errors in the data
can occur. The infrared wavelength used cannot be seen by the human eye.
The PROFIBUS ILM therefore indicates when other light sources subject it to an illegally high infrared radiation
by lighting up the red "ERROR" LED. The user can also configure a switch to activate the signaling contact in
this situation.
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