12-2
RLC-4 V1.79 Copyright © 1998 Link Communications Inc. 9/18/98
Analog Input Lines:
The analog input lines are designed to read a voltage on one of two ranges: 0 to 5 volts, or 0 to 25
volts. You can select which range is used with the DIP switches labeled "Divider" located near the
"Options" connector. If the switch is on, you will be using the 0 to 25 volt scale, because it
"divides" the input voltage by 5. No matter which way the switch is set, most of the discussions
about the analog inputs will send the voltage input as being 0 to 5 volts since the voltage divider is
in hardware and the software doesn't know whether the divider is turned on or not. If you do have
the divider turned on, just remember that 5 volts to the software means 25 volts to you.
The other four DIP switches (other than the “Divider” switches) are labeled “Power”. When turned
on, they connect a pullup resistor to the numbered analog input line. The pullup resistor is needed
when interfacing to LM335 temperature sensors, but not when interfacing most other analog
voltages.
The RLC-4 provides a lot of flexibility in how it reads the analog input lines. Unfortunately, this
also makes things a little bit complicated. Charts have been provided so that you can use the input
lines without understanding how all of the math works, but if you have a unusual application, the
RLC-4 can handle that as well. The following paragraphs explain what some of the options are
when reading analog lines. The way you select from those options is described with the commands
themselves.
"Resolution" refers to how many digits after the decimal place you want to know about.
Temperature is usually read to the nearest degree, zero digits after the decimal. When reading a
battery or power supply's voltage, you probably want to hear more than "thirteen volts" or
"fourteen volts", something like "thirteen point six volts". To obtain this kind of reading, you
would specify one digit after the decimal point.
The "conversion ratio" specifies how the controller interprets the voltage it detects. It could also
be called the "scale", “conversion ratio” or "meter faceplate". If you are measuring wind speed, you
may want a reading that varies from zero to 100 MPH. If wind direction, zero to 360 degrees. If
pH, zero to 14. If temperature, way below zero to a hundred degrees or more. No matter what
scale you want the reading to use, the voltage going into the analog line must be between 0 volts
and 5 volts (or 0 and 25 with the voltage divider on). If you have a sensor to measure something in
the physical world that can provide a voltage that varies between 0 volts and 5 volts (or 25...), you
can set up the conversion ratio to handle it. The point of this discussion is to make it clear that the
controller does not care what the real-world quantity is, it just needs a variable voltage and the
proper conversion ratio, and it can handle it. You tell the RLC-4 what the conversion ratio is by
specifying two points:
The first is what real world quantity would cause the sensor to output 0 volts. In many
cases this is zero. For example, if you are using a small motor with a propeller to detect
wind speed and the wind is not blowing, you will get 0 volts. But what about temperature?
If our sensor outputs 0 volts for zero degrees, how would we get negative temperature
readings? The analog lines can only accept positive voltages. The answer is that we use a
temperature sensor that outputs about 2.5 volts at zero degrees, less than that when below
zero and more than that when above zero. So our first conversion point for temperature
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