User`s guide

7 ELECTRONICS AND INTERFACING
This short, simple introduction to the electronics most often needed by digital I/O
board users covers a few key concepts.
IMPORTANT NOTE
It cannot be stated often enough to those unfamiliar with the
82C55, WHENEVER THE 82C55 IS POWERED ON OR RESET,
ALL PINS ARE SET TO HIGH IMPEDANCE INPUT.
The implications of this fact 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 state after power on
or reset, pull all pins either high or low through a 2.2K resistor.
7.1 PULL UP & PULL DOWN RESISTORS
Whenever the 82C55 is powered on or reset, the control register is set to a known
state. That state is mode 0, all ports input.
When used as an output device to control other TTL input devices, the 82C55 applies
a voltage level of 0V for low and 2.5V-5V for high. It is the output voltage level of
the 82C55 that the device being controlled responds to.
The concept of output voltage for an 82C55 in input mode is meaningless. Do not
connect a volt meter to the floating input of an 82C55. It will show you nothing of
meaning. In input mode the 82C55 is in 'high Z' or high impedance. If your 82C55
was connected to another input chip (the device you were controlling), the inputs of
that chip are left floating whenever the 82C55 is in input mode.
If the inputs of the device you are controlling are left to float, they may float up or
down. Which way they float is
dependent on the characteristics
of the circuit and the electrical
environment; and may be unpre-
dictable! This is why it often
appears that the 82C55 has gone
'high' after power up. The result
is that your controlled device
gets turned on! That is why you
need pull up/down resistors.
Shown here is one 82C55 digital
output with a pull-up resistor
attached.
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1 INTRODUCTION
This manual provides information on PCI-DIO24, PCI-DIO24H and
PCI-DIO24H/CTR3 digital I/O boards and accessories. The manual is organized into
separate sections for those aspects of a product which are unique. Some issues, such
as simple programming and electronic interfacing are applicable to all of the digital
boards.
The PCI-DIO24 is a single 82C55 digital I/O chip interfaced to the PCI bus, with all
its I/O lines accessible through the board's 37 pin connector. The I/O pins of an
82C55 are CMOS TTL level.
PCI-DIO24H is a high drive, 24 line digital I/O board. The control register which
sets the direction of the I/O ports is identical to an 82C55 in mode 0 (see 82C55 data
sheet). The I/O pins are high drive TTL capable of sourcing 15mA and sinking
64mA.
PCI-DIO24H/CTR3 is a PCI-DIO24H with an 82C54 counter added. The 82C54 is a
10MHz down-counter chip with three 16 bit counters. The functions of the counter
(Input, Gate and Output) are brought out to those pins which are used for bus power
access on the PCI-DIO24 and PCI-DIO24H. The PCI-DIO24H/CTR3 uses 8 I/O
addresses. The lower four are occupied by the 82C55 digital I/O chip emulation and
the upper four are occupied by the 82C54 counter timer chip.
This manual provides information on programming the 82C55 in mode 0. Those
wishing to use the 82C55 in modes 1 or 2, or who wish to program the 82C54 counter
on the PCI-DIO24H/CTR3, must procure a data book from Intel Corporation Litera-
ture Department.
All these products are supported by Universal Library programming library. As an
owner of this product, you are entitled to the latest revision of the manual and soft-
ware. Just call with your current revision numbers handy, and request an update be
sent to you.
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