Floating Dio State; Dio Channel Configured For Low Dio State With External Load - National Instruments DAQPad-6508 User Manual

Floating DIO State

© National Instruments Corporation
DAQPad-6507/6508
82C55A
Figure 3-7. DIO Channel Configured for Low DIO State with External Load
Example:
The switch is set in the low DIO state, which means all DIO lines are pulled
low. If you want to pull one channel high, follow these steps:
1. Install a load (R
L
greater the current consumption and the lower the voltage (V).
2. Using the following formula, calculate the largest possible load to
maintain a logic high level of 2.8 V and supply the maximum sink
current (I).
⇒ R
V = I * R
L
V = 2.2 V is the voltage across R
I = 28 µA + 10 µA is the 2.8 V across the 100 kΩ pull-up
resistor and 10 µA from 82C55A leakage current
Therefore:
= 5.7 kΩ is the 2.2 V / 38 µA
R
L
This resistor value, 5.7 kΩ, provides a minimum of 2.8 V on the DIO line
at power up. You can substitute smaller resistor values but they draw more
current, leaving less sink current for other circuitry connected to this line.
The 5.7 kΩ resistor reduces the amount of a logic low sink current by
0.8 mA with a 0.4 V output.
The DIO lines are not pulled high or low by your DAQPad device in this
configuration. Use an external 100 kΩ resistor to pull the line to the state
you want.
+5 V
100 kΩ
GND
). Remember that the smaller the resistance, the
= V / I, where:
L
3-11
Chapter 3 Signal Connections
R
L
Digital I/O Line
L
DAQPad-6507/6508 User Manual