Intek RheoVac 940 - Manual (Page 13)

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Figure 10 - 4-20mA Output Circuit
Ñ Figure 9 provides the RheoVac wiring
detail for the 10 channel 4-20 mA outputs.
Table I provides the appropriate
connection identification. As stated, the
output signals are not isolated from each
other and therefore the 4-20 mA receiver
channels must be independently isolated.
A typical current output circuit is shown in
Figure 10. The current control circuitry
works by controlling the (
! ) side return
current through Q1 which returns current
through the isolated, but circuit-common,
ground (
L
1
). When the 4-20mA output is
configured as active, the (+) terminal is
connected to a common 35 to 40Vdc
supply (V
unreg
). The circuit “A” is
jumpered to the (+) output terminal and
“B” is jumpered to the (
! ) terminal. For
passive mode, the “A” is not connected,
the “B” is jumpered to the (+) terminal,
and the (
! ) terminal is connected to
isolated circuit ground ( L
1
).
The remaining outputs - RS232/422 and status, are all ( ! ) terminated to isolated circuit
ground. A single chassis or earth ground wire should be connected to the minus of any one
and only one of the receiving devices. This prevents high or noisy common-mode “floating”
potentials between the isolated transmitter and grounded transducer circuits. Do not connect
a ground wire to each output. Again, the 4-20mA, RS232/422 and status outputs are all
common to each other and should be connected to isolated input cards.
Ò The status output located on field wiring terminal, JP7, is a digital 0-15Vdc output. This output
will go low in the event of a fault or power loss. The remaining JP7 outputs are not normally used
with the RheoVac monitor and should be left unconnected. If a non-standard option has been
ordered there will be additional notes in the
SECTION 6.3 SPECIAL INSTRUCTIONS.
Ó Mount the transmitter enclosure. Install conduit such that all seals are watertight and rigidly
secure. A separate external power switch is recommended to shut the equipment off during
outages. When the vacuum system is on-line, do not turn off power to the RheoVac monitor
unless you are preparing to take the probe out of the pipe.
Ô Choose a path for the transducer to transmitter cable conduit. Route the transducer interface
cable through the conduit (See Figure 9). The cable is labeled at both ends. DO NOT CUT
OR SPLICE THE CABLE, AS THIS WILL DESTROY THE LABELING AND MAY
AFFECT THE INSTRUMENT CALIBRATION. Pull the cable through the conduit starting
at either end; coil up the remaining length outside the transmitter or transducer enclosure, or
in a cable junction box.