Rockwell Automation T8448 Trusted TMR Zone Interface Module - 40 Channel - Manual (Page 13)

Trusted
TM
Module T8448
Issue 14 Apr 10 PD-T8448 13
1.9. Output Switch Structure
The Zone Interface 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 OFIU 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.
Figure 3 Output Switch Structure
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.
1 The lower switches are depicted as N.C. (Normally
Closed), and are controlled by the “upstream” neighbouring FIU. 2
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
3 (unlike
electromechanical relays for example).
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. The structure of each OFIU output is shown
below:
1 Their “home” FIU.
2 The home FIU, supplies an independent control signal for the “downstream” FIU FSS.
3 For an un-faulted transistor.