require you to use the shorting wires supplied with the board to
short inputs to outputs for I/O testing.
3
A voltage divider takes advantage of the fact that the voltage across one of the resis-
tors in a circuit is proportional to the voltage across the total resistance in the circuit.
The trick to using a voltage divider is to choose two resistors with the proper propor-
tions relative to the full scale of the digital input and the maximum signal voltage.
The phenomena of dropping the voltage proportionally is often called attenuation.
The formula for attenuation is:
For a given attenuation, pick a handy resistor
and call it R2, then use this formula to calcu-
late R1.
R1 = (A - 1) * R2
For example, if the signal varies between 0
and 20 volts and you wish to measure that with
an analog input with a full scale range of 0 to
10 volts, the Attenuation is 2:1 or just 2.
2 = 10K + 10K
10K
The variable Attenuation is the proportional
difference between the signal voltage max,
and the full scale of the analog input.
Attenuation = R1 + R2
R2
Digital inputs also make use of voltage dividers, for example, if you wish to measure a
digital signal that is at 0 volts when off and 24 volts when on, you cannot connect that
directly to the CIO-CTR digital inputs. The voltage must be dropped to 5 volts max
when on. The Attenuation is 24:5 or 4.8. Use the equation above to find an appropri-
ate R1 if R2 is 1K. Remember that a TTL input is 'on' when the input voltage is
greater than 2.5 volts.
IMPORTANT NOTE: The resistors, R1 and R2, are going to dis-
sipate all the power in the divider circuit according to the equation
Current = Voltage / Resistance. The higher the value of the resis-
tance (R1 + R2) the less power dissipated by the divider circuit.
Here is a simple rule:
For Attenuation of 5:1 or less, no resistor should be less than 10K.
For Attenuation of greater than 5:1, no resistor should be less than
1K.
The CIO-TERMINAL has the circuitry on board to create custom voltage dividers.
The CIO-TERMINAL is a 16" by 4" screw terminal board with two 37 pin D type
connectors and 56 screw terminals (12 - 22 AWG). Designed for table top, wall or
rack mounting, the board provides prototype, divider circuit, filter circuit and pull-up
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