ROC364 Instruction Manual
3-11
Input and Output Modules
Rev Jun/05
3.4.6 Discrete Input Isolated Module
A schematic representation of the field wiring connections to the input circuit of the Discrete Input
Isolated module displays in Figure 3-12.
NOTE:
The Discrete Input Isolated module is designed to operate only with discrete devices
having their own power source, such as “wet” relay contacts or two-state devices providing an
output voltage. The module is inoperative with non-powered devices.
The Discrete Input Isolated module operates when a field device provides a voltage across terminals B
and C of the module. The voltage sets up a flow of current sensed by the module that, in turn, signals the
ROC electronics that the field device is active. When the field device no longer provides a voltage,
current stops flowing and the DI module signals the ROC electronics that the device is inactive.
A 10-ohms scaling resistor (R1) is supplied by the factory and accommodates an external voltage (V
o
) of
11 to 30 volts dc. 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 optimum value of R1 displays in Figure 3-12. For best efficiency, R1 should be scaled
for a loop current (I) of 3 milliamps.
–
+
SELF-POWERED
DISCRETE DEVICE
3.3K
N/C
R1 + R
W
+ 3.3K = LOOP RESISTANCE = 4.5K OHMS MAX
V
O
= VOLTAGE FROM DISCRETE DEVICE = 11 TO 30 VDC
I = LOOP CURRENT = 3 mA TYPICAL
TO OPTIMIZE SCALING RESISTOR R1:
R
W
= RESISTANCE OF FIELD WIRING
– R
W
– 3.3K
R1 =
V
O
V
O
– 1
I
R
W
B
C
A
–
+
DOC0144A
R1=10
DI ISO
I
Figure 3-12. Discrete Input Isolated Module Field Wiring
3.4.7 Discrete Output Source Module
A schematic representation of the field wiring connections to the output circuit of the Discrete Output
Source module displays in Figure 3-13.
The Discrete Output Source module is designed to operate only with non-powered discrete
devices, such as relay coils or solid-state switch inputs. Using the module with powered devices
may cause improper operation or damage to occur.
The Discrete Output Source module provides a switched voltage across terminals B and C that is derived
from internal voltage source V
s
. A field device, such as a relay coil, is energized when the ROC
electronics provides a voltage at terminals B and C. When V
s
is switched off by the ROC electronics, the
field device is no longer energized.
