Rockwell Automation T8449 Trusted TMR 24V dc Valve Monitor Module - 40 Channel - Manual (Page 15)

Trusted
TM
Module T8449
Issue 13 Apr 10 PD-T8449 15
used to control field devices. The remaining 20 channels always remain de-energized and are used to
monitor feedback taking advantage of the channels built-in voltage measurement capability.
Throughout this document, the 20 channels used to control field devices will be referred to as “control
outputs”. The remaining 20 channels used to monitor feedback will be referred to as “feedback inputs”.
Each control output is paired with a feedback input. This I/O pair represents the entire interface for a
single field device.
The Module Configuration section contains more details on the physical layout of the control outputs
and feedback inputs.
1.11. Valve Testing
In order to test a valve, a signal is sent by the application program to the module. The module will then
change the state of the valve control output until either the positional feedback indicates that the valve
has moved, or a timer expires. The results of the test are supplied to the application program as a set
of time values, each of which corresponds to a limit switch change. The application program can use
the time values to determine the operational health and characteristics of the valve.
1.11.1. Positional Feedback
Monitoring the position of the valve is accomplished by measuring the voltage of the feedback channel
that is connected to one or more limit switches. Where multiple limit switches are available, a
mechanism external to the module is used to combine the various limit switch positions into a single
voltage measurable by the module. The feedback path utilizes the voltage-monitoring portion of an
output channel circuit. As such, the output circuit itself is never energized.
Thresholds will be applied to the voltage level to produce a discrete state (in much the same way as in
a TMR 24Vdc Digital Input module). Each discrete state will fully describe the positions of all connected
limit switches. If the number of limit switches allows, additional states may be used to provide end-of-
line monitoring.
1.11.2. Normal Operation
Normal operation of the control output is the same as that of a 24Vdc Digital Output channel. In all
cases, the normal operation of a control output takes precedence over any active testing.
Just like a regular 24Vdc Digital Output module, all control outputs are considered safety critical. Their
safe state is De-energized (Off).
When the control output is commanded to change states (i.e. from Energized/On to De-energized/Off),
the module will record the time difference from the commanded change to one or more changes in the
positional feedback levels. The recorded times will be available to the user application.