User manual

Chapter 3 Signal Connections

Power Connections

Pins 49 and 99 of the I/O connector are connected to the +5 V supply

from the PC power supply. These pins are referenced to GND and can

be used to power external digital circuitry. This +5 V supply has a 1 A

protection fuse in series. This fuse is self-resetting. Simply remove the

circuit causing the heavy current load and the fuse will reset itself. For

more information on these output pins, see Output Signals in

Appendix A, Specifications.

Power rating 0.5 A per pin at +5 V ±10%

Warning: Under no circumstances should these +5 V power pins be connected

directly to ground or to any other voltage source on the PC-DIO-96/PnP or

any other device. Doing so may damage the PC-DIO-96/PnP and the PC.

National Instruments is

NOT liable for damage resulting from such a

connection.

Digital I/O Power-up State Selection

You may want to power up the PC-DIO-96/PnP’s digital I/O lines in a

user-defined state. The PC-DIO-96/PnP facilitates user-configurable

pull-up or pull-down. Each DIO channel is connected to a 100 kΩ

resistor and can be pulled high or low using jumper W1. You can use

W1 to pull all 96 DIO lines high or low. However, if all lines are high,

you may want to pull some lines low. To do this properly, you must

understand the nature of the drive current on those lines and adhere to

TTL logic levels.

High DIO Power-up State

If you select the pulled-high mode, each DIO line will be pulled to VCC

(+5 VDC) with a 100 kΩ resistor. If you want to pull a specific line low,

connect between that line and ground a pull-down resistor (R

) 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. Use the largest possible

resistor so that you do not use more current than necessary to perform

the pull-down task.

Also, make sure the resistor’s value is not so large that leakage current

from the DIO line along with the current from the 100 kΩ pull-up

resistor drives the voltage at the resistor above a TTL low level of

0.4 VDC.