Chapter 3
Signal Connections
©
National Instruments Corporation
3-11
PCI-DIO-96/PXI-6508/PCI-6503 User Manual
However, ensure the resistor 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 3-5.
DIO Channel Configured for High DIO Power-up State with External Load
Example:
By default, all DIO lines are pulled high at power up. To pull one channel
low, complete the following steps:
1.
Install a load (R
L
). Remember that the smaller the resistance, the
greater the current consumption and the lower the voltage.
2.
Using the following formula, calculate the largest possible load to
maintain a logic low level of 0.4 V and supply the maximum driving
current:
V = I × R
L
R
L
= V/I,
where
V = 0.4 V; Voltage across R
L
I = 46
μ
A + 10
μ
A; 4.6 V across the 100 k
Ω
pull-up resistor
and 10
μ
A maximum leakage current(except lines PC0 and
PC3)
therefore
R
L
= 7.1 k
Ω
; 0.4 V/56
μ
A
This resistor value, 7.1 k
Ω
, provides a maximum of 0.4 V on the DIO line
at power up. You can substitute smaller resistor values to lower the voltage
or to provide a margin for V
cc
variations and other factors. However,
smaller values draw more current, leaving less drive current for other
circuitry connected to this line. The 7.1 k
Ω
resistor reduces the amount of
logic high source current by 0.4 mA with a 2.8 V output.
The maximum leakage current on most lines is 10
μ
A. The maximum
leakage current on the PC(0) and PC(3) lines is 20
μ
A.
DIO Board
Digital I/O Line
82C55
100 k
Ω
GND
R
L
+5 V