Remote-processing RPC-52 Manuel d'utilisateur télécharger pdf (Page 30)

ANALOG I/O CHAPTER 10

Page 28

measur ement r ange is 1 to 5V, the count range is

reduced by 20% to 818. The constant K is computed as:

K = 5/818

K = .006188

The above equation is assuming the 4-20 ma loop is

returning a value that represents a range of 0 to 5V. As

in the previous example, if pressure were measured:

K = 200/818

K = .24449

There is one addition factor. Since the lowest value read

is 1 V, this offset is subtracted from all readings. A 1 V

offset is 1/5 of 1023 counts, or 205. The program line

then becomes:

200 A=.006112*(AIN(N)-205)

Note that if the current loop line breaks, a negative value

is returned.

CALIBRATION

The A/D comes factory calibrated for a 0 to 5V input.

This span can be changed by adjusting R1. You can

adjust the span to 5.12V. This is useful when the input

is 0 - 5V and you want to know when the input is over-

range.

To calibrate or adjust the voltage reference:

1. Connect the voltmeter ground to any even

numbered pin on J1. Make sure there ar e no

other connections to the analog ground.

2. Connect the voltmeter ' + ' lead to U5, pin 6.

3. Adjust R1 for 5.00 VDC or other voltage as

desired. Do not exceed 5.2 V.

ANALOG OUTPUT

The two analog output channels shar e the PW M outpu ts

at the digital port. Analog output 0 is controlled by

PWM 0 and output 1 by PWM 1. When a PWM output

is used by an opto channel, it should not be used as an

analog output.

To make sure PWM is not used by an opto channel, set

the following jumpers:

W6[1-2] PWM 0

W7[1-2] PWM 1

One channel may be used as a PWM output while the

other is used as an analog output. Both outputs are

independently programmable as to duty cycle but not

frequency.

Analog output is generated from the PWM outputs by

simply placing a low pass filter in the outp ut. T he result

is an output with a 10K series resistance. The filter

capacitor is 10 Mfd. T hese values p rovide r easonable

ripple filtering and response over the PWM' s output

frequency.

The 10K output resistance may be high for some

devices. Check the device you are interfacing to. If the

impedance is less than 1 Meg ohm, and certainly 100K,

then the accuracy and maximum output will be affected.

If the full 5 volt range is not necessary, then the lower

impedance may not be a problem. The output may be

buffered using an external OP amp.

To make sure the output voltage is at the desired level,

the output can be connected to the analog input. Then,

by reading the voltage the output can be adjusted to the

desired level. The maximum output voltage will be

reduced when using this method, however.

The output voltage is proportional to the PWM cha nnel' s

duty cycle. Thus, the output voltage will change in steps

of 1/255, or approximately 20 mv/ step. This effectively

makes the analog output act like an 8 bit D/A converter.

The accuracy of the output depends upon the accuracy of

the + 5 V supply. When the supply is exactly 5 volts,

the output will go nearly to + 5 and ground at the

extrem e range s. W hen the output is loaded, + 5 volts

may not be reached. With the output loaded at 100K,

output voltage is reduced by about 10%.

Programming

The PWM statement is used to set the output voltage.

CONFIG PW M can be used to set the output and

frequency on power up.

The output voltage is proportional to the duty cycle:

Vo = Vcc - (Vcc * duty cycle / 255)

For example, a duty cycle of 80 produces an output of

3.43 volts. This assumes the power supply (Vcc) was

exactly 5 volts and the output was not loaded. The

following example r amps both outputs:

100 FOR N=0 TO 255

110 PWM 0,N

120 PWM 1,N

130 NEXT

140 GOTO 100

The outputs will ramp from + 5 volts to ground then

return to + 5V to repeat the cycle.

1 2 ... 25 26 27 28 29 30 31 32 33 34 35 36

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Remote-processing RPC-52 Manuel d'utilisateur Page 30

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