Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 36)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 36
If the relay load current is less than the minimum, an extra load resistor is required as described in the
previous section.
Hazardous area outputs
Digital outputs are not intrinsically safe or non-incendive. Separate power-limiting devices should be
fitted to satisfy the regulations in force. For EEx ‘d’ devices, mounted in sealed explosion-proof
housings, direct connection is allowed because the device housing provides the protection.
Device Characteristics
Digital output line monitoring algorithms expect the output current and voltage to be predictable. This is
adequate for linear loads (e.g. resistive, inductive), but many field devices do not present simple loads.
Low current loads
Minature relays, LEDs, lamps and PLC inputs usually have insufficient current to satisfy the channel
diagnostics. These should be fitted with load resistors as described above.
Some low current loads can have unusual effects on the diagnostic tests due to nonlinearity. For
example, a filament bulb has a very low resistance when cold and may appear as a short circuit when
first energised. On firmware versions from TUV 3.5, the output drive logic will drive a short circuit with
more determination, whilst protecting the output switches from overheating.
Another example is a load which appears open circuit until adequate voltage is applied; this can
frustrate ramp tests for open switches (fault code 0x63nn). This has been seen with LED lamps. The
solution is to assume that the load itself has no effect on the impedance seen by diagnostic tests and
to add parallel resistors to provide the minimum current at all times.
Dynamic loads
These loads draw irregular current and include strobe beacons, pulsing sirens and other devices that
draw intermittent current. These may temporarily draw more than the maximum rated current or less
than the minimum load, or even both. When energised, the channel state will not stay continuously in
state 4 but will switch to 3 (low load), 5 (short circuit) or even 6 (field fault). This can cause
discrepancies which will be indicated as slice faults, when one slice measures a transient at a different
point to the other two slices. All transitions in and out of fault states are indicated in the I/O module
event logs.
If state 3 is seen, add load resistors as above.
If state 5 is seen, ensure that the digital output module has firmware installed to TUV release 3.5 or
later. This is indicated by a firmware version 130 and a manufacturing build number 38 for T8448,
T8449, T8451 or T8461, 39 for T8471 and 18 for T8472. This release improved handling of short
circuit detection. If release 3.5 output module firmware still reports short circuits on loads apparently
within its rating, replace or refit the device to smooth out its demand peaks.
If state 6 is seen, the load is dynamically ‘tri-stating’ or otherwise changing the impedance, which
causes the voltage to change. Add a load resistor as for low current loads, to ensure the voltage
remains steady. The output module’s voltage sensing circuit causes the line voltage to be lifted to the
state 3 region (see page 27) during a true open circuit. A T8461 from hardware build I and a T8451
after build H (‘LV2’) have an impedance of about 66K to a reference point two volts below field supply
voltage. A T8461 before hardware build I has an impedance of about 33K to one volt below supply
voltage.
Maintaining line monitoring with devices that go high impedance when de-energised is difficult; the best
solution is to add a bleed resistor across the load to lower the open circuit voltage to the state 2 region.
Suggested resistors are given in the ‘Termination’ section of each output module product description
(these sections discuss the termination of unused outputs, but the reason is the same). Note that there
must still be adequate power available through any I.S. barrier to allow the device to be energised.