Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 28)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 28
Line Monitoring
Outputs do not have threshold templates. Instead, the state of an output is determined automatically by
measurement of its voltage and current with few user-definable settings. The state is the module’s
measurement of the output condition and is not the same as the output command. Each of the three
slices of an output module makes a judgement on the state of the output. The majority vote is reported
to the application through the STATE board. If one slice reports a different state, the channel is
reported as discrepant with a 70nn series slice fault and a bit is set in the DISCREP channels.
The states are described in the sections below. They refer to the thresholds shown in this table. Note
that the values for T8451 also apply to T8448 and T8449. The field supply voltage measurement is
after a semiconductor drop through the Group Failsafe Switches (GFSS), so the figures below have
been raised by 0.5 volts to reflect the true value. The Off and On thresholds are measured from the
field supply voltage downwards, so they will vary with the field supply voltage (and a volt has been
added). The measurement is negative because the FET switches in the output circuits are high-sided
and thus measurements must be referenced to the top rail (see ‘Power Groups’ above). The No-Load
threshold is user-adjustable and is explained in a later section.
T8451 to
rev. G T8451 from
rev. H (‘LV2’)
T8461 T8471
Minimum field supply voltage 11.5V 11.5V 12.2V 34.5V
No load threshold per slice
(adjustable) 10mA 10mA 10mA 10mA
Off threshold (below supply) -15.3V -15.3V -16.7V -87V
High On threshold (below supply) -1.3V -4.5V +2.3V -27.5V
Low On threshold (below supply) -4.5V -5.5V -5V -82.5V
No field supply voltage (state 1)
This state is reported when the measured field supply voltage for the power group is below the
minimum threshold. It is possible to run a module with a low supply voltage. The switching circuits will
work, and the output will be energised and de-energised by these switches. However, all channels will
report state 1, which effectively means there is no line monitoring.
Output de-energised/off (state 2)
This occurs when an adequate field supply voltage is present and the voltage is below the Off
threshold. This demonstrates that the output has successfully switched off and there is no residual
voltage on the loop.
Open circuit in field wiring or load (state 3)
This occurs when:
• for energised outputs, an adequate field supply voltage is present and the measured current is
below the no-load threshold. This is a common source of 7000 series slice discrepancy faults,
if one slice is starved of current by the passive sharing between slices.
• for de-energised outputs, an adequate field supply voltage is present, loop current is below the
no-load threshold and the channel voltage is between the low and high On thresholds.