Remote-processing RPC-52 Bedienungsanleitung PDF herunterladen (Seite 29)

CHAPTER 10 ANALOG I/O

Page 27

data point.

The program below takes about 2. 4 mS per data point.

10 DIM A(254)

20 FOR X=0 TO 254

30 A(X) = AIN(0)

40 NEXT

This next program saves data above MTOP . M TOP was

previously set by CONFIG MTOP to 30000. However,

if you have 128K or m ore RA M, you can P OKE into

segment 1 or higher. It takes approximately 3.6 mS per

data point and is not affected by the mem ory locatio n to

save to.

10 A = 30000

20 FOR X=0 TO 999

30 POKE W0,A,AIN(0)

40 A=A+2

50 NEXT

Data is retrieved using the PEEK W command

Reducing n oise

An input channel can appear to be noisy (change

readings at random) if unuse d inputs are allowed to float.

To minimize noise, connect all unused inputs to ground.

A high impedance is, by definition, sensitive to voltage

pickup. Noise is minimized by running wires away from

AC power lines. A low impedance voltage source helps

to reduce noise pick up. Shielded cable can help reduce

noise from high impedance sources. Make sure the

shield is not used for pow er gr ound. Using the sh ield

for power ground defeats its purpose.

Data logging on a timer tick

Some applications require that data is read at fixed

intervals. The ONTIC K construct can be used to take

data in intervals from 0.0 1 to 327 seconds. T he exam ple

below takes 1 sample per second until 100 samples have

been obtained.

10 DIM A(100)

20 ONTICK 1,500

30 REM THE REST OF YOUR PROGRAM

40 REM CONTINUES

80 GOTO 30

500 A(N) = AIN(3)

510 N=N+1

520 IF N = 100 THEN ONTICK 0,500

530 RETI

MEASURING HIGHER VO LTAGES

Voltages higher than + 5V can be measured 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.

Maximum

Input Voltage Resistor

6 20K

12.5 150K

24 380K

The following formula is used to determine the series

resistance necessar y for a m aximum voltage input:

Rs = Vi * 20000 - 100000

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.

NOTE: When an input voltage exceeds + 5 or is less

than 0 volts, other channel values are affected.

Converting analog measureme nts

Inputs can be converted to engineering units of

measurement by performing scaling calculations in the

program. The AIN function retur ns values from 0 to

1023. To change these numbers into something more

meaningful, use the following formula:

var = K * AIN(n)

n is the analog channel to read. K is the scaling

constant. K is obtained by dividing the highest number

in the range of units by the maximum AIN count (1023).

Example 1: To measure the results of an A/D

conversion in volts and the voltage range is 0 to 5V,

divided 5 by 1023 to obtain K.

K = 5/1023

K = .004887

Your program could look something like:

1000 C = .004887 * AIN(N)

Example 2: You want to measure a 0 to 200 PSI

pressure transducer with a 0 to + 5V output. Divide 200

by 1023 to obtain the constant K.

K = 200 / 1023

K = .1955

The code can then look like:

1000 B = .1955*AIN(0)

Measuring 4-20 mA current loops

Current loops is a convenient way to transmit a value

and still assure the integrity of the signal. If the line

should break, a 0 volt (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.

Current loop r eadings are conver ted to engineer ing units

by performing scaling as described earlier. Since the

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