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Siemens TX-I/O™ Module system – Functions and operation CM110561en_08
Building Technologies Digital output functions 2018-10-25
5.4 Maintained contact, bistable
(BO Bistable NO / NC)
On/off switching of devices and loads.
Interruption of the power supply or the bus communications: the last state of the relays
is maintained (bistable relays). In addition, the module supports setting of a backup
state as well as a time delay before this backup state becomes effective.
– Light control
– Control of subsystems with uninterruptible operation
• The function is supported by module type TXM1.6RL only
• The function requires one I/O point
• The module has N/O contacts only
• The relay contacts are especially adapted for light applications
Process value
I/O status LED
Relay contact
The signal operation can be selected: Relay (Default), Relay_NA (NA = normally active).
In case of bus failure (Masterdown), where the DC 24 V module supply is intact, the
following backup states can be parameterized:
– No (Same effect as Value Off)
– Value (On / Off)
– Keep Same value as before the fault (= default)
– Delay before the backup state becomes effective:
(0...6553s, default 120s, steps of 0.1s).
Parameterizing can be made during operation.
A powerfail DC 24 V always induces a bus failure - see above.
In case of powerfail AC 24 V (Powerdown AC 24 V), the following backup states can
be parameterized:
– No (Same effect as Value Off)
– Value (On / Off)
– Keep Same value as before the fault (= default)
Parameterizing can be made during operation.
This function is available with relay type TXM1.6RL only.
It supports changing to backup values even when the module supply is lost, not only in
case of lost communications.
• For this purpose, the AC 24 voltage on the bus is monitored. This monitoring is fast
enough to make the relays take their backup positions before the DC 24 V supply
breaks down.
• As the relays are bistable, they stay in the backup position even without DC 24 V
supply.
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