Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 6)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 6
The T8842 Versatile Field Termination Assembly (VFTA) provides individually configurable channel
circuits for a T8431 and other low voltage 40 channel modules. It also provides three-wire connection
(24V supply, signal and zero volt reference). It requires a VFTA cable TC-211, 212, 511 or 512. Circuit
configuration is given in PD-8842. Fuse ratings may be chosen up to a total of 3A per group of eight
channels. For analogue inputs, a link is required in position E, a 50mA fuse in position A to limit the
current loop, and a 315mA fuse in position B. If the device requires more power, fit separate fused
terminals alongside the VFTA.
Four-wire devices also require a power 0V return as well as a signal reference. For power less than
315mA, fit both zero volt wires to terminal 3. If separate fuse terminals are required, also fit separate
zero volt terminals.
Other VFTAs have existed but were all fixed versions of the T8842.
Current sourcing and sinking, active and passive loops
The T8431 and T8432 all expect to measure the voltage on the channel with respect to the zero volt
reference. In the current loop circuits above, current flows into the system circuit, down through a
precision resistor (thus creating a voltage drop), and into the zero volt reference. The input channel
voltage above the zero volt reference is therefore a measure of the loop current.
In the diagram above, an active or field powered loop drives current round the loop. It may have its own
local power supply, or be powered from the system through VFTA or separate terminals.
A passive or system powered loop takes a little power from the system supply to drive a variable loop
resistance which controls the loop current. This is the reason that current loops are usually 4 to 20mA;
the spare 4mA is used to power the resistance control.
All these devices are current sourcing, that is, they supply current to the system’s channel connection.
Some devices are current sinking; they take current from the channel connection. These devices
cannot be directly connected to T8431 or T8432 inputs, because the system circuit would have to
measure voltage from the supply rail with an upside-down circuit, as shown below. This circuit cannot
be used.
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
24V
Channel
0V
System circuit Passive field loop Active field loop