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7. Theory of Operation
8. Program Example
For older CR7s with Instruction 29, the Device (parameter 2) specifies what type
of synchronously addressed peripheral is to be addressed. The Device code for an
SDM-CD16D is 2.
For Instruction 29 only (older CR7s), the Card parameter (parameter 4) specifies
which 725 Excitation Card is being used for the control port signals. The Reps
parameter does not advance beyond the specified Card, requiring another
Instruction 29 for each 725 Excitation Card used.
On power-up, all SDM-CD16D outputs are off /LO.
The SDM-CD16D is a synchronously addressed peripheral. C2 and C3, driven
high by the datalogger, initiate a cycle. While holding C3 high, the datalogger
drives C2 as a clock line and C1 as a serial data line. The datalogger shifts out a
data bit on C1 (LSB first) on the falling edge of the C2 clock. The SDM-CD16D
shifts in the C1 data bit on the rising edge of the C2 clock.
The first eight bits clocked out represent the SDM-CD16D address. If the address
matches the SDM-CD16D's address, the SDM-CD16D is enabled. If enabled, the
next 16 bits are shifted into the SDM-CD16D, each bit controlling one port, the
first of which controls output 1.
When the 16 control bits are clocked in, C2 is held high while C3 is pulsed low
then high to latch the control bits. The datalogger then lowers both C3 and C2 to
complete the cycle.
The example is written for the CR10/10X Measurement and Control Module. The
program concepts presented are the same for the CR23X, 21X and CR7
dataloggers with minor changes in the program code.
In this example, the SDM-CD16D is used to control the temperature between 23°C
and 28°C in each of five greenhouses. In each greenhouse the SDM-CD16D
controls a heating unit, a refrigerating unit and an air mixing fan according to the
following conditions:
Heating unit:
Activate when temperature <23.5°C.
Deactivate when temperature >25.5°C.
Cooling unit:
Activate when temperature >27.5°C.
Deactivate when temperature <24.5°C.
Mixing fan:
Activate whenever the heating or cooling units are activated.
Activate for 5 minutes out of every 15 minutes.
The program assumes that the temperature measurements have been made, and
that the average temperature for each greenhouse is computed and stored in input
locations 1 to 5. For further information on loops and input location indexing,
please refer to your datalogger manual.
Input location assignments are as follows:
Input
Input
Location
Location Label
1..5
Temp #1..#5
User Guide
Description
Avg. temp. greenhouse 1..5
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