Measurement Computing PC104-DIO48 - Manual (Page 16)

5 ELECTRONICS AND INTERFACING
This brief introduction to the electronics most often needed by digital
I/O board users covers the following subjects:
O Pull-up/pull-down resistors
O Transistors
O Power MOSFETs
O Solid State Relays
O Voltage dividers
O Low pass filters for digital inputs
O 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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