FloBoss 407 Instruction Manual
3-14 Rev 5/00
3.4.5 Discrete Input Source Module
A schematic representation of the field wiring connections to the input circuit of the Discrete Input
Source module is shown in Figure 3-11.
CAUTION
The Discrete Input Source module is designed to operate only with non-powered
discrete devices such as “dry” relay contacts or isolated solid-state switches.
Use of the module with powered devices may cause improper operation or
damage.
The Discrete Input Source module operates by providing a voltage across terminals B and C that is
derived from internal voltage source V
s
. When a field device, such as a set of relay contacts, is
connected across terminals B and C, the closing of the contacts completes a circuit which causes a flow
of current between V
s
and ground at terminal C. This current flow is sensed by the DI module, which
signals the ROC/FloBoss electronics that the relay contacts have closed. When the contacts open,
current flow is interrupted and the DI module signals the ROC/FloBoss electronics that the relay
contacts have opened.
A 10-ohm scaling resistor (R1) is supplied by the factory and accommodates a source voltage (V
s
)
of 11 to 30 volts dc. The source voltage is the input voltage to the ROC/FloBoss. However, it is
desirable to optimize the value of R1 to reduce the current drain from the source or reduce the heat
generated in the module due to high source voltage. The formula for determining the value of R1 is
given in Figure 3-11. For optimum efficiency, R1 should be scaled for a loop current (I) of 3
milliamps.
PULSE DEVICE
ROC-POWERED
V
S
= SOURCE VOLTAGE FROM MODULE = 11 TO 30 VDC
R1 + R
W
+ 3.3K = LOOP RESISTANCE = 4.5K OHMS MAX
– R
W
– 3.3K
R
W
= RESISTANCE OF FIELD WIRING
TO OPTIMIZE SCALING RESISTOR R1:
I = LOOP CURRENT = 3 mA TYPICAL
R1 =
I
I
R
W
V
S
– 1
B
C
–
A
N/C
+
R1=10
3.3K
DI SRC
DOC0143A
Modified
Figure 3-11. Discrete Input Source Module Field Wiring
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