5.8.4 Controlling While Scanning Differences
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The new numbering scheme is necessary because, unlike the Model 370, standard
temperature curves will be loaded into the Model 372 prior to shipment.
5.8.4 Controlling While
Scanning Differences
When a measurement input channel is assigned to an output for controlling, the
Model 370 automatically takes control of the reading sequence. It forces the scan
dwell time to 5 s for the channel used for controlling, and one second for other read-
ing channels, independent of autoscan being turned on or off. When autoscan is
enabled, the Model 370 automatically scans back to the channel used for control in
between the scanning of the non-control channels, using the previously mentioned
dwell times. When autoscan is disabled, the Model 370 automatically scans to the
channel used for controlling and dwell for 5 s, the scan back to the manually selected
reading channel and dwell for one second.
After re-evaluating the autoscanning requirements, we decided to get rid of this
forced autoscanning behavior and put the power back to you, the user. What this
means is that when autoscanning is enabled, the Model 372 will still scan back to the
control channel(s) in-between each reading, however it will not use forced dwell
times of 1 and 5 s. Instead, it will use your user-specified scan dwell, pause, and filter
settings. While this provides greater flexibility, care must be taken to ensure, for
example, that the dwell times are not too long on non-controlling channels to main-
tain stable PID control.
When autoscanning is disabled, the Model 372 will not automatically switch back to
the channel used for controlling if a non-control channel is manually selected. How-
ever, if this behavior is desired, autoscan can be enabled with a dwell time of 5 s being
assigned to the channel used for control and 1 s for the non-reading channel.
5.8.5 Hardware
Differences
One of the most significant hardware differences between the Model 370 and
Model 372, besides the dedicated control input, is that the analog 1 voltage output of
the Model 370 has been replaced with a warm-up heater current source. The Model
370 provides a ±10 V voltage source output with 100 mA maximum current, provid-
ing 1 W into a 100
)
heater. The Model 372 provides a +630 mA current source, pro-
viding up to 10 W into a 25
)
or 50
)
load.
If you require the warm-up heater to function as a voltage source, then a resistor
must be added across the warm-up heater output terminals to convert the current
source to a voltage source. In addition, the warm-up heater maximum current must
be set appropriately so that 100% current output equates to 10 V measured across
the conversion resistor.
The following example shows how to calculate the conversion resistor value when
the user load is of a high impedance. The warm-up heater resistance is set to the 25
)
setting in order to maximize the current output (0.63 A) and subsequently, maintain
maximum resolution.
R = V / I
R = 10 V / 0.63 A
R = 15.87
)
16
)
is a common resistor value that would bring the full scale voltage to 10.08 V. In
addition, the resistor must have a power rating high enough to handle the amount of
current.
P = I * V
P = 10 V * 0.63 A
P = 6.3 W