Output Switch Structure - Rockwell Automation ICSTT-RM280N-EN-P Manual

1. Description

1.9. Output Switch Structure

The Digital Output Module provides a TMR switch topology where the load is driven by a
total of three fully monitored, fail safe (6 element) switch channels, one physically resident
on each FIU in the Module. Any single switch or entire slice failure is designed to leave two
of the three fail safe switch channels operational to power the load.
The upper switches as shown in Figure 3 are denoted as N.O. (Normally Open), and are
controlled by the FIU on which they are physically resident.
depicted as N.C. (Normally Closed), and are controlled by the "upstream" neighbouring FIU.
Note: In this context, N.O. is defined as being in the off state in the absence of control signal power, and
similarly, N.C. is the on state in the absence of control signal power. These switches are constructed from
enhancement mode MOSFETs and are both guaranteed to be off in the absence of Module power to create
gate voltage signals to bias them on
The reason that the lower switches are specified to be on in the absence of control signal
power is to allow two channels to power the load should an entire slice fail. Even if an entire
slice fails, the surviving output circuits will carry the necessary control.
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Their "home" FIU.
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The home FIU, supplies an independent control signal for the "downstream" FIU FSS.
3
For an un-faulted transistor.
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Trusted TMR 24Vdc Digital Output Module – 40 Channel
Figure 3 Output Switch Structure
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(unlike electromechanical relays for example).
Issue 14
The lower switches are
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Rockwell Automation Publication ICSTT-RM280N-EN-P
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