Specifications

Digital I/O Functions 6-9
SETPOINT
In the preceding example, the low and high alarm limits are used to specify
a hysteresis value around a desired setpoint. To change the desired
setpoint, both the low and high alarm values must be changed. In this type
of controller operation, the Read Data (RD) or New Data (ND) commands
will read out the actual value of the process variable.
The D1000 modules provide a means of downloading a setpoint value
without affecting the desired hysteresis by using the Setpoint (SP) com-
mand. The Setpoint command is used to load the desired control value
into the output offset register (see Figure 2.1). The value in the output
offset register is always added to the data derived from the sensor input.
For instance, if the sensor data is +00100.00 and the output offset register
contains +00050.00, a Read Data command will yield an output of
+00150.00. The Setpoint command loads a value into the offset register to
null out the sensor data. If the command $1SP+00100.00 is given to a
module with address 1, the effect of the command is to load the output
offset register with -00100.00. An RD command will now result in the
deviation of the input data from the downloaded setpoint value.
A careful look at Figure 2.1 will reveal that the alarm limits are checked
after the output offset is added the input data. To construct a controller
using the SP command, the high and low alarms must be loaded with the
hysteresis values referred to the deviation from the setpoint value. In the
oven controller example, the hysteresis was set to ±5°C from the desired
control temperature of 100°C. When using the SP command, the high limit
would be set to +00005.00 and the low limit would be set to -00005.00 to
get the same hysteresis affect. The Latching modes of the alarm limits are
used in the same manner as previously described.
Let’s look at the oven controller again using the Setpoint command. The
desired oven temperature is 100°C. This time we’ll use the SP command
to load the 100°C value into the temperature module. As before, we would
like a hysteresis band of ±5°C from the nominal temperature of 100°C. In
this case, set the low limit to -00005.00 latching and the high limit to
+00005.00. The high and low limits are now used solely to define the
hysteresis band. If the oven temperature is low, say 90°C, the resulting
deviation from the setpoint of 100°C is -10°C. This value exceeds the low
limit and the LO alarm control output is turned on to activate the heater.
The latched LO alarm will stay on until the measured temperature exceeds
105°C. At this point the deviation from the setpoint is greater than ±5°C,
the value loaded into the high limit. When the high limit is exceeded, the
latched LO alarm output is turned off, turning off the heater. The control
action is identical to the controller described in Figure 6.6.