Rockwell Automation T8442 Trusted TMR Speed Monitor - Manual (Page 11)

Trusted
TM
Module T8442
Issue 8 Apr 10 PD-T8442 11
The quad redundant relay output stage is located on the T8891 SOFTA and is arranged as a fault
tolerant structure. The four force guided relay contacts are connected in a series/parallel arrangement
to ensure that any single failure cannot affect the desired load state. Diagnostics detect the current in
each series leg of the relay structure. The force guided auxiliary contacts are monitored to confirm
contact transition.
Diagnostics are used extensively to ensure that covert failures are detected and result in a fail-safe
reaction. For example, a fault within Speed Input Slice A, as shown in Figure 1 is localised to that input,
allowing Processor Slice A and Output Quads 1 to 4 to continue operation, i.e. the input operates
1oo2D whilst the remainder of the system continues to operate 2oo3.
The notation Slice A, Slice B and Slice C for individual channels in the triplicated design is used
throughout this document.
The notation Quad 1, Quad 2, Quad 3 and Quad 4 for individual channels in the quad design is used
throughout this document.
1.2. Speed Monitor Sub-Assembly Architecture Overview
This section shows the physical aspects of the T8442 Speed Monitor Architecture. It shows the
separate sub-assemblies that make up the Speed Monitor and how they interconnect.
The block diagram of the T8442 Speed Monitor Module is given in Figure 2 and shows that the T8442
system is made up of six separate sub-assemblies:
HIU (Host Interface Unit)
SFTU (Speed Field Termination Unit)
SFIU (Speed Field Interface Unit)
FPU (Front Panel Unit).
T8846 SIFTA (Speed Input Field Termination Assembly)
T8891 SOFTA (Speed Output Field Termination Assembly)
The T8846 SIFTA and T8891 SOFTA are external DIN rail mounted pieces of equipment and are
considered integral to the overall T8442 system.