Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 10)

Trusted
TM
AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 10
T8433 Termination Options
The T8433 Trusted
TM
Isolated Analogue Input module has individually isolated inputs. These measure
0-22mA. The module has a three-wire connection for supply, reference and signal; the extra wires
mean that there is only room for 20 channels. The module input circuit is completely isolated and
requires an external supply of 4.5 to 8 volts, derived in the circuit below from the loop.
The diagram above shows the input circuit using a T8833 FTA, a T8433 isolated analogue input
module and a 60 channel cable TC-601, 603, 701 or 703. 60 channel cables are required due to the
extra wires needed.
The current loop is measured across the 100R resistor (creating a voltage drop of 0.4 to 2 volts for 4 to
20mA). However, the isolated input circuit also needs a power source of between 4.5 and 8 volts; in the
above circuit it is provided by the zener diode using the current in the loop from the transmitter. All
circuit permutations need to allow for current measurement and channel power on opposite sides of
the common reference wire.
The T8433 FTA above allows for field powered current sourcing inputs, i.e. from devices that have
their own power supply and drive the current loop towards the channel. The device needs to have at
least 7.5 volts of spare loop drive available.
If the device is passive, i.e. it controls the current flow through itself but does not supply power to the
loop, it can be powered from an individual isolated supply as shown below.
This clearly requires an extra stage of terminals between the FTA and the field. Note that although it is
possible to use one power supply to power several loops, they would then no longer be isolated from
each other.
TB1 A
TB1 B
100R
7V5
SUPPLY
REFERENCE
SIGNAL
+
TB1 A
TB1 B
100R
7V5
SUPPLY
REFERENCE
SIGNAL
+
PSU
+ -