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Siemens TX-I/O Engineering and installation guide CM110562en_13
Building Technologies Principles of electrical design 2019-05-15
10.5 Cables for the island bus (DC 24 V)
A critical factor when planning the layout of remote sub-islands is the admissible length
of the island bus cables.
In the case of the AC 24 V supply voltage, long distances are easy to overcome by
ensuring that the sub-island has its own transformer (see the wiring diagrams in
Section 6.5).
With the island bus, however, the cable length is limited by capacitance and by voltage
drop.
• The total capacitance of all cable sections in an I/O island must not exceed 4.0 nF.
– All sections must be added together.
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Key:
= I/O row = Island bus cable
• The voltage difference between any two points on the island bus conductor CS
must not exceed 0.5 V (Section 10.2).
– The cable length between the most distant points of the island counts.
– The voltage drop occurs mainly in the bus cables between the I/O rows or sub-
islands.
– The return route for the supply current is via system neutral (⊥).
– The bus signal currents on conductor CD are negligible.
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Key:
= I/O row = Island bus cable
Limiting factors