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Defining A Data Logging Problem
drive can remain in the power-off state during periodic measurement activity.
Loop 2 only needs to
power up for disc drive access.
8.4 DATA STORAGE REQUIREMENTS
To compute the total storage required for a data logging session, determine the "worst case" record size for
the group that stores the most readings.
In addition to the channel readings, each scan stores the time,
date, and group number, adding three numbers to each scan. The storage requirement for a complete scan
is 1 byte plus 8 bytes for each number stored. Multiply this figure by the total number of scans+3 to find
the final size of the data file.
Suppose the largest group in the setup stores 15 channels of readings. This works out to 18 numbers per
record, or a record size of 1+18*8 or 145 bytes. If there are § groups in the setup, and each group takes
33 scans, the final size of the file is 145%(5%*33+3) or 24,360 bytes.
If the data file is to be stored on the disc, it is possible to compute the number of scans that will be taken
before the internal file ElIFFEF must be flushed to the disc.
To compute this figure, subtract
3500+(270*number -of -groups) from available memory in the HP-71, divide that figure by the record
size, and round down.
Some analysis of the order of group execution will be required to estimate the
actual time between disc writes.
For instance, if there is just one group which is scanned every minute,
and the record size is 145, an available memory size of 4756 bytes would yield a disc write about every 32
minutes. If the disc is connected to an a.c. charger, this would provide a reasonable recharge interval
between disc writes.
8.5 DATA LOGGING EXAMPLE
Now let's look at an application of the L.I15 program. In the darkroom problem of section 7 the
MOMITORE
program
was
used
to
monitor
the
temperature
of
a
darkroom
and control
a
constant-temperature bath within the darkroom. Let's continue with this application, this time using the
data logging program. In this section the system will be described. Subsection 9. 14, "Setup Example," will
describe the use of the "ETLIF program to create the setup file for the system. Subsection 10.2, "Log
Example," will continue the example by showing how the L.[Iz program is used to run the system.
The LIS program allows you to log the status of a system, and to control the system using limit tests.
Consider a production darkroom with a controlled-temperature bath that must be maintained within one
degree of 25°C. As in the example of section 7, two type T thermocouples are used to monitor the
temperatures. The room-temperature thermocouple is connected to channel 3 of the HP 3421A Data
Acquisition/Control Unit, and the bath-temperature thermocouple is connected to channel 4. In addition,
the bath heater is controlled by a relay, which in turn is controlled by actuator channel 0 of the
HP 3421A. Two HP 82401 A HP-IL Interfaces are connected to the HP-71 using an HP 82402A Dual
HP-IL Adapter. The HP 3421A is located on loop 1, and a disc drive on loop 2, as shown in figure 8-1.
Keeping the disc on loop 2 will keep the disc drive batteries from being discharged during the frequent
access to the HP 3421A.
8-4
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