D8 Series User’s Guide
Chapter 2: Installation
37
Control and Alarm Output Connections
Typically control and alarm outputs use external optically-isolated solid state relays (SSRs). SSRs
accept a 3 to 32VDC input for control, and some can switch up to 100 Amps at 480VAC. For larger
currents, use silicon control rectifier (SCR) power controllers up to 1000 Amps at 120 to 600VAC.
You can also use SCRs and a Serial DAC for phase-angle fired control.
The control and alarm outputs are open collector outputs referenced in the D8’s common. Each
output sinks up to 60mA DC to the controller common when on.
NOTE!
Control outputs are sink outputs. They sink current when the output is on.
Connect them to the negative side of solid state relays.
Figure 2.19 shows sample heat, cool and alarm output connections.
Cool Output
Alarm Output
+5VDC
+
-
+
-
+
-
SSR
SSR
SSR
TB50 or TB18
Heat Output
Figure 2.19
– Sample Heat, Cool and Alarm Output Connections
Heat Output
Cool Output
Alarm Output
Common
+
-
SSR
TB50 or TB18
- PS +
+
-
SSR
+
-
SSR
Figure 2.20
– Output Connections Using External Power Supply
CPU Watchdog Timer
The CPU watchdog timer constantly monitors the microprocessor. It is a sink output located on
TB50 terminal 6 or TB18 terminal 3. The output can be connected to an external circuit or device
to monitor whether the controller is powered and operational. The output is on (low) when the
microprocessor is operating; when it stops operating, the output goes off (high).
Figure 2.21 and Figure 2.22 on the following page show the recommended circuit for the watchdog
timer output for the TB50 and the TB18.
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