M E A S U R E M E N T S Y S T E M S I N T E R N A T I O N A L
Page 72
MSI-3750 Digital Weight Indicator
◆
User Guide
Programmable Logic Controllers (PLCs) are in common use for machinery
control. The 3750 can interface to PLCs with the relay outputs or with the logic
output on set point 8. Figure 6 shows one possible way of interfacing to a PLC.
More sophisticated PLCs are able to take data directly from RS-232 and inte-
grate the data into the control process. See the RS-232 option section for details.
Logic Input to PLC
+5V from PLC
Isolated Interface to a PLC
NO
CM
NC
1A
SMD FUSE
Relay
Logic Gnd from PLC
Figure 6
LOGIC OUTPUT (SET POINT 8)
Set Point 8 is unique from Set Points 1-7. The output consists of a logic output
capable of driving standard TTL loads. As the output is in reality 4000B series
CMOS devices, it may be necessary to provide an external pull-up resistor in
certain cases. Values of 1k
Ω
to 4.7k
Ω
usually will suffice. There are two
complementary outputs as shown in Figure 7. The TTL Hi output goes high
when the set point is tripped. The TTL Low output goes low when the output is
tripped. The logic output ground must be connected to provide a reference level.
Output level is nominally 5V but can vary from 4.8 to 5.2V.
TTL LO
GND
TTL HI
Setpoint 8
Isolated
Ground
Isolated
+5V
Figure 7
Besides providing a direct logic output to PLCs or other controllers, the logic
output can be used to drive external relays. Figure 8 shows an example of the
logic output driving an external N-channel MOSFET which can turn on a high-
power relay.
WARNING
Do not connect any voltage outputs to the Logic Output Pins. They are
protected for up to 15V only. The Logic Output should be interfaced only
with logic inputs or base/gate connections to transistors.
Summary of Contents for MSI-3750
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