Campbell Scientific SDM-CD8S 8 Channel Solid State DC Control Module - Manual (Page 14)

SDM-CD8S 8 Channel Solid State DC Control Module
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-CD8S 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.
7. Theory of Operation
The SDM-CD8S 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-
CD8S shifts in the C1 data bit on the rising edge of the C2 clock.
The first 8 bits clocked out represent the SDM-CD8S address. If the address
matches the SDM-CD8S's address, the SDM-CD8S is enabled. If enabled, the
next 16 bits are shifted into the SDM-CD8S, each bit controlling one port, the
first of which controls OUT 1. (Only the first 8 bits are used by the SDM-
CD8S.)
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.
8. Program Examples
8.1 Control Temperature and Fans – CR1000
In this example, the SDM-CD8S is used to control the temperature between
23
° and 28°C in 2 greenhouses. In each greenhouse the SDM-CD8S 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 the temperature measurements have been made, and the
average temperature for each greenhouse is computed and residing in the
appropriate variable
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