Fundamentals of a
Programmable Controller
Chapter 2
2-7
Message Storage
The third area of memory, message storage, begins after the end statement
in the user’s program. Two alphanumeric characters can be stored in a
word. Messages are entered into memory from either a 1770-T3 terminal
or a peripheral device.
Messages are displayed on a 1770-T3 terminal or a peripheral device each
time a message is required. The messages are activated through program
control by programming specific instructions in the ladder
diagram program.
Input Modules
The input modules of a programmable controller have four functions:
termination
indication
conditioning
isolation
Termination
The input provides terminals for the field wiring coming from the sensing
devices on the machine.
Indication
The input of most modules also provides a visual indication of the state of
each input terminal with LED indicators. The indicator is on when there is
a voltage applied to it terminal. The indicator is off when there is no
voltage applied to its terminal. Since the indicator reveals the status of its
terminal, it’s usually called an input status indicator.
Input indicators are only associated with terminals used for wiring sensing
devices to the input section. The terminal that’s used to provide a ground
for the sensing circuits has no indicator.
Conditioning
Another function of input modules is signal conditioning. Voltage levels
used at the machine are usually not compatible with the voltage levels used
within the programmable controller. The input modules receives the
electrical signal from the machine and converts it to a voltage level
compatible with the programmable controller’s circuitry.
Isolation
The input isolates the machine circuitry from the programmable
controller’s circuitry. Isolation helps protect the programmable controller’s
circuitry from unwanted and dangerous voltage levels that may occur
occasionally at the machine or in the plant’s wiring system.
