Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 32)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 32
Termination
Powered digital output terminations are very simple, with minor additions to suit each module type.
The above diagram is complete for T8448 and T8449 outputs, T8451 and T8461. T8471 requires 330K
resistors (0.25W) in parallel with the load and also on unused channels, to provide termination to the
diagnostic tests, so they can distinguish between an open circuit and a stuck-on fault. These are fitted
to the T8870 field termination assembly (FTA). The T8871 FTA, used for the T8472, has transient
protection components.
The zero volt return is taken back to the supply outside the module. A connection is made to the
module for measurement of the field supply voltage.
Digital outputs do not require fusing because the modules dynamically control the output and protect
against short circuit. In the event of a short circuit, digital outputs will de-energise until turned off and
on again or the processor Reset button is pressed, when the module will try to energise the output
again.
T8842 VFTA
The T8842 Versatile Field Termination Assembly (VFTA) provides individually configurable channel
circuits and is essential for the T8448 and T8449, but could also be used for the T8451 and T8461. It
also provides three-wire connection (24V supply, signal and zero volt reference). It requires a VFTA
cable TC-209, 210, 509 or 510. Circuit configuration is given in PD-8842.
For digital outputs, the resistors play no part in the circuit, which is shown by the bold lines. The 2A
fuse is only used as a link and is not a circuit protection.
Field
Power
TBFPn
0V
Channel 1
B
Precision Resistor
Signal A
0V
Field
Terminals
TBn
Signal
TBGn
Power Resistor
C
D
F
E
Fuse Field Power
3
2
1 1
2
3
4
m
2
2A
Output
module
Field Supply
Load
for reference only