Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 30)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 30
No-load (Open Circuit) Threshold
Triplicated Modules 8448, 8449, 8451, 8461, 8471
(See later for 8472, 8473)
The No-Load threshold settings are a common cause of confusion, leading to copying of settings from
other configurations without understanding. For this reason they are covered in more detail here.
The no-load threshold is a current in milliamps per channel slice below which the channel is declared
open circuit.
The diagram above shows the FET switches in a single output channel (in simplistic detail). The output
channel is switched by the three slices of the triplicated circuit, each of which measures the current
through one leg of the circuit. The total channel current is thus the sum of the current through the three
legs.
However, the circuit does not share the current evenly. It is quite possible for the current in one leg to
be twice that of another leg. For that reason, the channel current at which a no-load is declared may be
four or five times the no-load threshold.
The no-load threshold is also used as a detection of cross-talk or noise on de-energised channels. If
there is more than the no-load threshold current on a de-energised output, then there is significant
leakage, and this is a serious problem in a safety system, to the point that the slice may be isolated by
turning it off. A combination of heavy switching loads and a low no-load threshold makes a false
positive on these diagnostics more likely. There is also a known cross-talk from channel 18 of an 8461
to some adjacent channels (see technical note TN20031), which will make the problem even more
likely if channel 18 has a heavy load.
The no-load threshold can be set for each power group, e.g.:
nlthresh = 3,5,10,15,20
sets a no-load threshold of 3mA per channel circuit leg on outputs 1 to 8, 5mA on channels 9 to 16,
10mA on 17 to 24, 15mA on 25 to 32 and 20mA on 33 to 40. Since the threshold can be set for each
power group, it is better to group similar power demands on each group (e.g. don’t mix sirens and
LEDs). Then apply a no-load threshold that is comfortably below the minimum expected slice current.
The default no-load threshold is 10mA. This will be suitable for any load greater than 50mA. It has
been common practice to specify nlthresh = 5,5,5,5,5 i.e. 5mA on all groups. This allows loads down to
about 25mA to be recognized properly, and usually recognizes loads down to 20mA if the current
balancing is good. However, with very heavy loads in the same group as LEDs and relays, there may
be problems with a 5mA setting. 10, 15 or 20mA are more appropriate for heavy loads; see TN20031
and Product Description PD-8082 section 2.3.10.
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