Emerson Fisher FLOBOSS 407, Instruction Manual

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FloBoss 407 Instruction Manual
3-8 Rev 5/00
3.4 CONNECTING THE I/O MODULES TO WIRING
Each I/O module is electrically connected to field wiring by a separate plug-in terminal block. In
addition, the ROC/FloBoss enclosures provide a ground bus bar for terminating the sheath on shielded
wiring. The following paragraphs provide information on wiring field devices to each type of I/O
module.
CAUTION
The sheath surrounding shielded wiring should never be connected to a signal
ground terminal or to the common terminal of an I/O module. Doing so makes the
I/O module susceptible to static discharge, which can permanently damage the
module. Connect the shielded wiring sheath to a suitable earth ground only.
3.4.1 Analog Input Loop Module
The Analog Input Loop module monitors either loop current or output voltage from field devices. The
module provides source power at terminal A for the loop. The AI Loop module operates by measuring
the voltage at terminals B and C. For current loop monitoring, scaling resistor R1 generates a voltage
across terminals B and C that is proportional to the loop current (I). A 250-ohm scaling resistor (R1) is
supplied by the factory (0.1%, 1/8W) to accommodate either 0-to-20 milliamp or 4-to-20 milliamp
current loop transmitters. This translates to a maximum operating input voltage of 5 volts dc, which is
the upper limit of the module.
When using a transmitter with a maximum current requirement different than 20 milliamps, R1 should
be scaled to achieve full scale deflection at 5 volts dc. The formula for determining a new value of R1
is given in Figure 3-2, where “I Maximum” is the upper end of the operating current range (such as
0.025 amps for a 0 to 25 milliamp device).
I
CURRENT LOOP
DEVICE
ROC-POWERED
I MAXIMUM
R1 =
TO SELECT PROPER VALUE OF R1:
V
S
= SOURCE VOLTAGE FROM MODULE = 11 TO 30 VDC, 25 mA MAX
5 VOLTS
DOC0153J
+T
B
C
–
A
+
R1=250
I LIMIT
AI LOOP
V
S
+
-
Figure 3-2. AI Loop Module Field Wiring for Current Loop Devices