Hardware Reference
In-Depth Information
output will return to high to indicate acceptance of the new key after a normal
debounce period; this two key rollover is provided between any two switches.
An internal register stores the last key pressed even after the key is released.
It should be noted that the LS-TTL-compatible outputs of the keypad encoder
chip are tri-state, thus permitting direct connection to a data bus. Furthermore,
the active-low Output Enable (/OE) input to the device can be used in a variety
of configurations which permit asynchronous data entry as well as synchronous
data entry and synchronous handshaking. Figure 9.11 shows how this can be
achieved.
Proximity detectors
Proximity detectors are required in a wide variety of applications - from sensing
the presence of an object on a conveyor to detecting whether a machine guard
is in place. Simple proximity detectors need consist of nothing more than a
microswitch and suitable actuator whereas more complex applications may
require the use of inductive or capacitive sensors, or even optical techniques.
Microswitches
A microswitch is a simple electromechanical switch element which requires
minimal operating and release force and which exhibits minimal differen-
tial travel. Microswitches are normally available in single-pole double-throw
(SPDT) configurations and can thus be configured as either normally open (NO)
or normally closed (NC).
The principal disadvantage of the humble microswitch is that it not only
requires physical contact with the object sensed but also requires a force of
typically 40 to 200 g for successful operation. Most common microswitch
types (including the popular V3 and V4 types) can be fitted with a vari-
ety of actuator mechanisms. These include lever, roller, and standard button
types. Metal-housed and environmentally-sealed micro-switches are available
for more demanding environments.
Reed switches
Reed switches use an encapsulated reed switch that operates when in the prox-
imity of a permanent magnetic field produced by an actuator magnet. Reed
switches are generally available as either normally open (NO) or changeover
types. The later may, of course, be readily configured for either NO or NC oper-
ation. Distances for successful operation (pull-in) of a reed switch are generally
within 8-15 mm (measured between opposite surfaces of the actuator magnet
and reed switch assembly). The release range, on the other hand, is generally
between 10 and 20 mm.
Inductive proximity detectors
Inductive proximity detectors may be used for sensing the presence of metal
objects without the need for any physical contact between the object and the
sensor. Inductive proximity switches can be used to detect both ferrous and
