Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 40)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 40
Pulse Outputs
Applications have occasionally required pulse-width modulated outputs. These may be used to control
valve positioners using open and close pulses. Other applications may require accurate control of
output pulse times.
The T8449 Valve Monitor module is normally used to add valve testing to an output drive. The output is
briefly switched to the opposite state and the feedback input channel is monitored by the module to
check that the valve has moved. If the valve does not reach the test target position within a time limit,
the test is cancelled and the output is reinstated.
If a test is requested which cannot succeed, the output will pulse for the time limit setting, with an
accuracy of about 7ms. This may be used to generate output pulses for pulsed applications. Each
‘valve test’ is configured and demanded by the application as required. If the output is energised, the
output is switched off for the ‘test’ time limit. If the output is de-energised, the output is turned on for
the ‘test’.
‘Tests’ are demanded by a rising edge on a Boolean output. The application must therefore run two
scans to turn the output off and on to demand a pulse. This is the fundamental limit to the pulse
frequency.
Four variables need to be connected to the T8449 I/O definition.
Output Boolean output. Signal to drive steady output state on or off. Wired to odd-numbered
DO_TEST channel.
DO_Test Boolean output. Signal to demand a pulse on a rising edge. Wired to next (even
numbered) DO_TEST channel after ‘Output’.
ETime Integer output. Pulse width in milliseconds 0 to 32767). Wired to odd numbered
ETM_EST channel (same channel number as ‘Output’).
End_State Integer output. A target state number for the test, one which should never occur on the
unused input (e.g. 5). Wired to next (even numbered) ETM_EST channel after ‘Etime’. This variable
should be given an appropriate initial value and does not need to be changed by the application.
The example below shows two channels configured for pulsing. These are controlling positive/opening
and negative/closing pulses to a valve positioner.
To demand a pulse, set ETime to the required width and set DO_Test from False to True.
A function block has been written to control the opening and closing pulses described above. This
provides the three dynamic outputs needed for the connections shown above. Its inputs are described
below.