Remote-processing RPC-210 Uživatelský manuál Strana 46

  • Stažení
  • Přidat do mých příruček
  • Tisk
  • Strana
    / 60
  • Tabulka s obsahem
  • KNIHY
  • Hodnocené. / 5. Na základě hodnocení zákazníků
Zobrazit stránku 45
ANALOG I/O BASIC SECTION 10
Page 10-4
K = 200 / 4095
K = .0488
The result is in PSI w hen used as follows:
1000 B = .0488*AIN(0)
The constant can be stored in RAM or Flash memor y.
This “ constant” can be chan ged to compensate for gain
differences between sensors and the A/D’s on different
boards. See AIN -3.BA S for a sam ple progr am. . T his
program uses the operating range of a hypothetical
transducer to com pute an offset and constant.
TIMED A/D CONVERSIONS
The ONTICK or ON IT R constr ucts may b e used to
make periodic conversions. ONTIC K is used for
relatively frequent conversions (. 10 times/ second) while
ONITR uses the real time clock to initiate a conversion
at a predetermined date/ time or longer interval.
Frequently, A/D conversions are a part of some other
timed process that checks general system status.
See AIN-2. BAS for a program that performs conversions
on a timed interval using ONTICK.
MEASURING HIGHER VOLTAGES
Voltages higher than + 2.5V or 5V are measur ed by
inserting a series resistor to the input. A resistor can be
connected directly to the STB-20.
The table below shows resistor values for typical input
voltages using the 0-2.5V range.
Maximum Input
Voltage
Series resistor
3.00 200K
6.25 1.5M
12.25 3.90M
Use the following formula to determine the series
resistance necessar y for a m aximum voltage input:
Rs = Vi * 200,000 - 1, 000,000 0 - 5V range
Rs = Vi * 400,000 - 1, 000,000 0 - 2.5V r ange
Rs is the resistor value in ohms in ser ies with the input.
Vi is the maximum input voltage. When the result of
your calculation is negative or zer o, a series r esistor is
not necessary.
Adding a series resistor increases the source impedance
beyond recommended values. A way to compensate for
this is to put a capacitor at the line at J1 . This
effectively makes an RC filter. A 0.001 mfd or greater
capacitor is adequate.
NOTE: When an input voltage exceeds the input range,
other channel values are affected.
Due to the high resistan ces involved, you may want to
use a voltage divider instea d. If you do, don' t forget to
include the 1M ohm resistor to ground (R11) in your
calculations.
Measuring 4-20 mA Current Loops
Current loops are a convenient way to transmit a value
and still assure the integrity of the signal. If the line
should break, a 0 volts (or nearly so) is returned.
A 4-20 ma current loop is converted to 1 - 5V by placing
a 250 ohm resistor across the input of the chan nel to
ground. Since the measurement range is 1 to 5V, the
count range is reduced by 20% to 3276
CALIBRATION
There are tw o refer ences on the R PC-210. 5V is
adjustable while 2.5 is not.
You should calibrate 5.00V reference when you need
accuracies greater than 5 counts (7.3 mV @ 5V range)
on any one channel. Using this reference returns A-D
readings that are accurate to ±4 counts on any one
channel and up to ±7 counts difference between any two.
5.000V Reference
R16 is used to adjust the maximum, or reference
voltage. Its location is shown in Figure 11-1. When
W11 is installed, A/D and D/A ranges are 0 - 5V. If
W11 is installed, W12 must be set to [1-2].
Norm ally, the reference is 5. 000 volts. You can adjust
R16 so the reference voltage is between about 4.75 and
5.25 volts.
This reference obtains its power from the RS-232 chip.
If the serial ports are heavily loaded, the reference may
not be adjustable to the desired voltage. In this situation,
Zobrazit stránku 45
1 2 ... 41 42 43 44 45 46 47 48 49 50 51 ... 59 60

Komentáře k této Příručce

Žádné komentáře