Chapter 3
Signal Connections
PCI-DIO-96/PXI-6508/PCI-6503 User Manual
3-12
ni.com
Low DIO Power-up State (PXI-6508, PCI-6503 Only)
If you select pulled-low mode, each DIO line will be pulled to GND
(0 VDC) using a 100 k
Ω
resistor. If you want to pull a specific line high,
connect a pull-up resistor that will give you a minimum of 2.8 VDC. The
DIO lines are capable of sinking a maximum of 2.5 mA at 0.4 V in the low
state. Using the largest possible resistance value ensures that you do not use
more current than necessary to perform the pull-up task.
Also, ensure the pull-up resistor value is not so large that leakage current
from the DIO line along with the current from the 100 k
Ω
pull-down
resistor brings the voltage at the resistor below a TTL high level of
2.8 VDC.
Figure 3-6.
DIO Channel Configured for Low DIO Power-up State with External Load
Example:
Set jumper W1 to low, which means all DIO lines are pulled low at power
up. To pull one channel high, complete the following steps:
1.
Install a load (R
L
). Remember that the smaller the resistance, the
greater the current consumption and the higher the voltage.
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:
V = I × R
L
R
L
= V/I
where
V = 2.2 V; Voltage across R
L
I = 28
μ
A + 10
μ
A; 2.8 V across the 100 k
Ω
pull-up resistor
and 10
μ
A maximum leakage current (except lines PC0 and
PC3)
therefore
R
L
= 5.7 k
Ω
; 2.2 V/38
μ
A
DIO Board
Digital I/O Line
82C55
+5 V
GND
100 k
Ω
R
L