National Instruments PCI-6023E - Manual (Page 53)

Chapter 4 Signal Connections
PCI-6023E/6024E/6025E User Manual 4-24 © National Instruments Corporation
Digital I/O Power-up State
♦
(PCI-6025E Only)
The PCI-6025E contains bias resistors that control the state of the digital
I/O lines PA<0..7>,PB<0..7>,PC<0..7> at power up. Each digital I/O line
is configured as an input, pulled high by a 100 k
Ω
bias resistor.
You can change individual lines from pulled up to pulled down by adding
your own external resistors. This section describes the procedure.
Changing DIO Power-up State to Pulled Low
Each DIO line is pulled to V
cc
(approximately +5 VDC) with a 100 k
Ω
resistor. To pull a specific line low, connect between that line and ground
a pull-down resistor (R
L
) whose value will give you a maximum of 0.4
VDC. The DIO lines provide a maximum of 2.5 mA at 3.7 V in the high
state. Using the largest possible resistor ensures that you do not use more
current than necessary to perform the pull-down task.
However, make sure the resistor’s value is not so large that leakage current
from the DIO line along with the current from the 100 k
Ω
pull-up resistor
drives the voltage at the resistor above a TTL-low level of 0.4 VDC. Figure
4-12 shows the DIO configuration for high DIO power-up state.
Figure 4-12. DIO Channel Configured for High DIO Power-up State with External Load
Example:
A given DIO line is pulled high at power up. To pull it low on power up with
an external resistor, follow these steps:
1. Install a load (R
L
). Remember that the smaller the resistance, the
greater the current consumption and the lower the voltage.
Board
Digital I/O Line 82C55
100 k Ω
GND
R
L
+5 V
PCI.book Page 24 Wednesday, September 16, 1998 9:09 AM