Chapter 3
Functional Details
All of the digital outputs and inputs are CMOS TTL (Transistor-Transistor Logic). TTL is an industry term that
describes a standard for digital signals which are either at 0 V or 5 V (nominal).
The voltages and currents associated with external devices range from less than a hundred mA at a few volts for
lamps etc., to 50 Amps at 220 volts for electric heaters or other high-load equipment. Attempting to connect
such loads directly to the CIO-DIO24H will damage the I/O chip. Heavy loads such as these require external
relays.
Digital Interfacing
In addition to load matching, digital signal sources often need to be filtered or "de-bounced". Refer to the
Guide
to Signal Connections
at
www.mccdaq.com/signals/signals.pdf
for information on digital interfacing.
82C55 emulation
The CIO-DIO24H emulates the 82C55 chip. The 82C55 emulation initializes all ports as inputs on power-up
and reset. A TTL input is a high impedance input. If you connect another TTL input device to the output, it
could be turned
on
or
off
every time the board is reset.
To establish a consistent TTL level at power-up, use resistors tied to 5V (pull-up) or ground (pull-
down). There are open locations for pull-up and pull-down resistor packs on the board.
Whenever an 82C55 emulation is powered on or reset, all pins are set to high-impedance input. Based on
standard TTL functionality, these inputs will typically float high, and may have enough drive current to turn on
external devices.
Consequently, 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 state after
power on or reset, pull all pins either high or low through a 2.2 K resistor.
Signal level control
The 82C55 digital I/O chip initializes all ports as inputs on power- up and reset. A TTL input is a high
impedance input. If you connect another TTL input device to the 82C55 it could be turned ON or OFF every
time the 82C55 is reset.
To prevent unwanted signal levels, and to drive all inputs on the device you are controlling to a known state
after power up or reset, install pull-up or pull-down resistors. If you are using the board to control items that
must be OFF on reset, install pull-down resistors.
A pull-up resistor pulls all digital pins up to +5 V (high logic level). A pull-down resistor pulls all digital pins
down to 0 V (low logic level).
The CIO-DIO24H has open locations where you can install a single inline package (SIP) resistor network for
each port. When installed, the SIP establishes either a high or low logic level at each of the I/O lines on the
port.
The SIP is made up of eight 2.2 K
Ω
resistors. One side of each resistor is connected to a single common point
and brought out to a pin. The common line is marked with a dot or line at one end of the SIP. The remaining
resistor ends are brought out to the other eight pins (see
).
Figure 8
15
Содержание CIO-DIO24H
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