Rockwell Automation T8444 Trusted TMR Pulse Generator and Monitoring Module - Manual (Page 14)

Trusted
TM
Module T8444
Issue 09 Apr 10 PD-T8444 14
1.9. Output Switch Structure
The outputs of the Pulse Generator and Monitoring 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 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.