Remote-processing RPC-330 Manuel d'utilisateur télécharger pdf (Page 37)

ANALOG INPUT SECTION 10

Page 10-3 RPC-330

40 NEXT

This next program saves data to RAM segment 1 in a

system with 128K of RAM. It takes approximately 2 mS

per data point and is not affected by the memory location

saved to.

10 A = 0

20 FOR X=0 TO 999

30 POKE W1,A,AIN(0)

40 A=A+2

50 NEXT

Data is retrieved using the PEEK W command.

Noise Notes

An input channel can appear to be noisy (change

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

To minimize noise (and increase accur acy), connect all

unused inputs to ground.

A high impedance input 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 power ground. Using the

shield for power ground defeats its purpose.

Wire pairs can also be twisted. 5-6 twists/foot provides

a reasonable amount of noise cancellation.

Noise is defined in this section as any random change

from a known input. The amount of noise you can

exp ect und er nor mal operatin g ci rc um sta nce s is ±3

counts for any input range.

One way to compensate for noise is to take a number of

samples and average the results. Taking 6 or more

samples would, in theory, cancel out any effects of

noise. A problem with this is noise tends to group

together. Taking 6 readings at one time might show no

change fr om the norm. Another 6 readings might be all

high. If possible, try to spread out readings over a

period of time (several seconds if possible).

Noise is, by definition, random. If you were to plot out

the deviations from a norm, it would roughly resemble a

bell shaped curve. Exper iments on the RPC-330 have

shown that 99% of the readings are within the ±3 count

reading. N oise read ings wer e made with all inputs

shorted to ground.

Temperature Measurement

Reference IC U14 outputs a voltage pr oportiona l to its

temperature.

Vo = 2.1(T + 273)

T = Vo/2. 1 - 273

T = Vc * .581428 - 273

Where T = Te mp er atu re in °C

Vo = Output voltage in mV

Vc = Count returned using AIN, 0 -

5V range

At 25°C the output voltage is approximately 630 mV, or

516 counts.

The output from U14 must be buffered. To measure

temperature, jumper H1[1-3]. Remove resistor R13.

Jumper H1[2-4]. Temperature is read at analog channel

100 T = AIN(0) * .581428 - 273

T returns the temperature in celsius.

The output from the temperature sensor varies from unit

to unit.

You can increase the output voltage from the

temperature sensor by lowering R12 and grounding H1

pin 5 (jumper H1-5 to H1-7).

Data logging on a timer tick

Some applications require that data is read at fixed

intervals. The ONTICK construct can be used to take

data in intervals from 0.01 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

1 2 ... 32 33 34 35 36 37 38 39 40 41 42 ... 47 48

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Remote-processing RPC-330 Manuel d'utilisateur Page 37

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