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12-9

101: Set Resolution For Analog Input

This command allows you to specify how many digits after the decimal point will be used when
reading each analog input line. If you want to read voltage to 1/10 of a volt, you specify one digit
after the decimal point. This decimal point is assumed in the other analog commands since there is
no good way to enter a decimal point on a DTMF pad. If you specify one decimal place with this
command, the other analog commands will assume that the numbers you enter have an assumed
decimal point one digit from the end. In other words, if you want to specify a value of 12 volts and
you are using one digit after the decimal point, you must enter 120. The descriptions of those
commands will explain this in more detail.

<101> l r

Parameters:

- 101 is the default command name.
- L is the analog input line number (1..4)
- R is the number of digits after the decimal point (0..3)

Explaination:

If you are not using any digits after the decimal point in your analog readings, you can skip the rest
of this section, as it explains how much accuracy you can expect for different types of readings.
Although you can specify as many as 3 decimal places, that doesn't mean that the analog to digital
converter has that much resolution. It accepts a voltage between 0 and 5 volts (at the processor,
voltage dividers on the I/O board and external to the RLC-4 can both affect the maximum voltage
that can be handled) and can differentiate 255 levels within that range, about 20mV per level.
LM335 temperature sensors output a voltage of 10mV per degree Kelvin (celsius - 273). This
corresponds to about 5mV (10mV * 5/9) per degree fahrenheit. If a temperature sensor is hooked
up with no voltage divider (the recommended way since it outputs less than 5 volts), this gives
approximately one degree resolution (it is actually slightly better than a degree). There is no reason
to try to read temperature with two decimal places when the actual resolution of the conversion is
only to the even degree (0 decimal places), it would just add meaningless digits. Even one digit
after the decimal point is pushing it - it would sound like you were getting 1/10 degree resolution
even though you would really only be getting about ½ degree resolution. We recommend reading
temperature to the nearest degree (0 digits after the decimal point).

Another example:

reading battery or power supply voltage. Since your battery probably supplies

between 12 and 14 volts, it can be read with the voltage divider on the I/O connector switched on
to provide a range of 0..25 volts. This is 5 times the range (25/5=5) that is presented to that
processor, so your precision is reduced to 1/5 of what it was (from 5mV to 25mV). You can
therefore read your battery voltage with one digit after the decimal point (100mV resolution) and
waste a little resolution, or use two digits after the decimal point (10mV) and exceed the resolution
of the converter.

Let us suppose that we are trying to read the voltage supplied by a 3 volt battery pack with 3 digits
after the decimal point (1mV resolution). Remember that the converter has only 20mV resolution