Switch Diagnostics - Rockwell Automation ICSTT-RM280N-EN-P Manual

Trusted TMR 24Vdc Digital Output Module – 40 Channel 1. Description
The structure of each FIU output is shown below:
Figure 4 Simplified Switch Circuit Diagram
A resistor provides a means of continuously monitoring the switch current. A signal
transistor is used to drive the gate of Switch 2. It provides Switch 2 with a negative gate
voltage, to minimize its on resistance, and serves to hold Switch 2 on in the event that the
secondary gate control loses power.
The Zener diode between the gate of Switch 2 and source is only required to protect the
gate from large voltage spikes on the drain that might capacitively couple through when
Switch 1 and Switch 2 are in the off state.
The resistors in series with the gate of Switch 1 and the signal transistor serve to protect the
drive logic in the event of a malicious switch failure. The pull-up resistors define the gate
voltages in the absence of power.
1.9.1.

Switch Diagnostics

During normal operation, Switch 1 and Switch 2 are maintained on. In this state, Switch 1
and Switch 2 exhibit a low resistance.
To determine the ability of the system to control the load via Switch 1 and Switch 2, their
gate voltages are modulated, one at a time. As the gate voltages are modulated, the
monitoring signals synchronously change in a predictable fashion. The local DSP analyses the
relative amplitude and phase of these small AC signals, to determine the on resistance and
threshold voltages of each switch.
The current to the load does not need to be completely interrupted in order to obtain a
level of confidence in the ability of the transistors to turn off. For the TMR switch
configuration in the on state, only one fail safe switch at a time needs to be modulated,
while the other two bear the load current.
Rockwell Automation Publication ICSTT-RM280N-EN-P Issue 14 9