Specifications

Digital I/O Functions 6-5
Alarm limit values are loaded into the module with the Low limit (LO) and
Hi limit (HI) commands. The limit values are stored in nonvolatile memory
so they will not be lost when power is removed. The HI and LO commands
are also used to specify whether the alarms are momentary or latching. If
an alarm is specified as momentary, the alarm is activated as long as the
alarm condition exists. The alarm output will turn off when the input is
within limits. A Latching alarm is activated when the specified limit is
exceeded and will remain on even if the input value returns within limits. A
Latching alarm can be turned off with the Clear Alarms (CA) command.
The HI alarm output is turned on (sinking current) when the measured
sensor input is greater than the high limit loaded in with the HI command.
The LO alarm output is turned on (sinking current) when the input value is
less than the stored low limit.
The alarm limit values may be read back at any time using the Read Low
(RL) or Read High (RH) commands.
ON-OFF CONTROLLERS
The alarm capabilities of the D1000 sensor-input modules may be utilized
to construct simple ON-OFF controllers that operate without host interven-
tion. In fact, since all the alarm information is stored in nonvolatile memory,
the module can act as a stand-alone controller with the communications
lines disconnected.
The simplest controller connection is to use a momentary alarm output to
control the process. A typical application would have a temperature input
module controlling a heater, as shown in Figure 6.5. To maintain a con-
stant temperature, set the low limit to the setpoint desired and specify the
alarm output to be momentary. Use the LO alarm output to control the
heater. If the temperature measurement exceeds the low limit, the heater
will be turned off. When the temperature goes below the limit, the LO
alarm output goes on, turning on the heater. The negative feedback action
of the control output will keep the temperature at the desired value. The
high limit is still available to activate an alarm or shut down the system if
the temperature goes out of limit.