Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 35)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 35
Terminating Resistors
Unused outputs should be commanded off/de-energised. These need a resistor to satisfy the
diagnostic logic, which is wired from the channel output to zero volts. This also prevents the need to
force channels to a healthy state.
The following values will drain sufficient leakage current to move the channel voltage down to the state
2 (off) range.
T8448, T8449, T8451 4K7 0.5W
T8461 10K 0.5W
T8471 330K 0.25W (already fitted to T8870 FTA)
T8472 33K 2W
Low current digital output loads may need the addition of a parallel resistor to increase the current
drain above the minimum load. This is common when the output drives miniature relays, matrix lamps
or high impedance inputs, e.g. to other systems. I/O module event logs often fill with field fault states,
and it is worth minimising the causes.
The minimum current should be set to five times the no-load threshold, to ensure that all three slices
have enough current to detect the load. Parallel resistance should be added to draw the extra current
required. If the field load is approximately half of the total, it is likely that a field open circuit will still be
detected because all three slices will detect insufficient load. If the resistor can be fitted locally across
the actual load, then the cable is still line monitored.
Volt-free outputs
All the termination details in the previous sections assume that the system powers the load with its own
supply. This allows full line monitoring for most devices. However, relays are often fitted in output
circuits to isolate systems.
If an output is safety related, the relay should be a certified safety relay. These have different designs
to satisfy requirements, and fall into two basic categories.
Force-guided contacts have a higher guarantee that the contacts will move. The contacts are joined
together and pushed directly instead of relying on their spring tension to return them to the de-
energised position. Spare contacts in this set could be used as auxiliary feedback contacts to prove to
the system that the contacts have moved, via a digital input. Force-guided contact relays are relatively
cheap and follow the EC Machinery Directive.
Some safety relays have dual or multiple redundant circuits with co-ordinating electronics. These are
designed to guarantee that the relay can move contacts to the de-energised position, for de-energise-
to-trip applications (e.g. ESD) and are more expensive. Electronics in the relay monitors the coil and
contact state and will prevent the relay from energising if a fault is detected. These add SIL-specified
integrity to de-energising applications but are a hindrance to energise-to-trip applications, e.g. F&G,
where a good quality basic relay would be better.
Output
module
Field Supply
Load
for reference only