Rockwell Automation T8451 Trusted TMR 24V dc Digital Output Module - 40 Channel - Manual (Page 13)

Trusted
TM
Module T8451
Issue 12 Apr 10 PD-T8451 13
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:
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.