Siemens SIMATIC NET PROFIBUS Owner's Manual page 354

Infrared Link Modul (ILM)
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Make sure that the infrared link cannot be disturbed, for example by "interrupting" the link with
obstacles, extraneous light etc.
When cascading with PROFIBUS ILM, only one segment with master nodes is permitted, the cascaded
segments must only contain slave nodes.
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Make sure that the two infrared transmission links do not interfere with each other, in other words
either the spatial arrangement of the modules (clearance) or a physical barrier (wall) must ensure
that each PROFIBUS ILM can only detect the transmission of its partner module and cannot detect
emissions from either of the modules of the other infrared link.
Note
When cascading, the delay times of the PROFIBUS ILMs must be taken into account. The delay times are
shown in Table 3 and must be included in the calculations during configuration.
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