Measurement-computing PC104-DIO48 Manuale Utente Scaricare il pdf (Pagina 16)

5 ELECTRONICS AND INTERFACING

This brief introduction to the electronics most often needed by digital

I/O board users covers the following subjects:

Pull-up/pull-down resistors

Transistors

Power MOSFETs

Solid State Relays

Voltage dividers

Low pass filters for digital inputs

Noise; sources and solutions

IMPORTANT NOTE:

WHEN AN 82C55 IS POWERED ON OR IS

RESET, ALL PINS ARE SET TO HIGH

IMPEDANCE INPUT.

The implication of this is that if you have output devices such as solid

state relays, they may be switched on whenever the computer is powered

on or reset. To prevent unwanted switching and to drive all outputs to a

known safe state after power-on or reset, pull all pins either high (to

+5VDC ) or low (to GND) through a 2.2K ohm resistor.

To install pull up/down resistor packs, refer to the following application

note.

5.1 PULL UP & PULL DOWN RESISTORS

This section deals with pull up/pull down resistors and 82C55 digital I/O

chips.

Whenever the 82C55 is powered-on or reset, the control register is set to

a known state; that state is mode 0, all ports set to inputs.

When used as an output device to control other TTL input devices, the

82C55 applies a voltage level of near 0V for low and near 5V for high.

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Measurement-computing PC104-DIO48 Manuale Utente Pagina 16