Unit installation

12-10
RLC-3 V1.80 Copyright © 1998 Link Communications Inc. 9/17/98
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> b l r
Parameters:
- 101 is the default command name.
- B is the I/O board number (1..2)
- L is the analog input line number (1..8)
- R is the number of digits after the decimal point (0..3)
Explaination:
If you don't care what happens if you exceed the resolution of the converter, skip all of the
following information and ignore this command. Please note that 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-3 can both affect the maximum voltage that can be handled) and can
differentiate 1024 levels within that range, about 5mV 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 1/2 degree resolution. We recommend reading temperature to the
nearest degree (0 digits after the decimal point).
Another example: reading battery voltage. Since your battery probably supplies between 12 and
14 volts, it can be read with the voltage divider on the I/O board 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 5mV resolution