Operational Power Control Systems 451
ing system. As a rule, each manufacturer has a unique set of component
numbers for its system. One manufacturer has a four-digit numbering
system for referencing components. The numbers are divided into dis-
crete component references and register references. A discrete compo-
nent is used to achieve on and off control operations. Limit switches,
pushbuttons, relay contacts, motor starters, relay coils, solenoid valves,
and solid-state devices are examples of discrete component references.
Registers are used to store some form of numerical data or information.
Timing counts, number counts, and arithmetic data may be stored in reg-
ister devices. All component references and register references are iden-
tified by a numbering system. Each manufacturer has a particular way
of identifying system components.Programming Basics
Programmable logic controllers are provided with the capabili-
ty to program or simulate the function of relays, timers, and counters.
Programming is achieved on a format of up to 10 elements in each hori-
zontal row or rung of a relay diagram, and up to seven of these rungs
may be connected to form a network. A network can be as simple as a
single rung, or can be a combination of several rungs, as long as there
is some interconnection between the elements of each rung. The left rail
of the ladder can be the common connecting element. Each network can
have up to seven coils connected in any order to the right rail of the
ladder. These coils can be assigned any valid number for identification.
The coil number can only by used once in the operational sequence. The
quantity of discrete devices and registers available for use depends on
the power or capacity of the system.
When programming a relay ladder diagram into a PLC, the discrete
devices and registers are placed in component format. Each component
in this case is assigned a four-digit identification number. The specific
reference number depends on the memory size of the system. In a low-
capacity system, number assignments could be 0001 to 0064 for output
coils, and 0258 to 0320 for internal coils. A system with a larger capac-
ity might use number assignments of 0001 to 0256 for output coils, and
0258 to 0512 for internal coils. Any coil output or internal coil can only be
used once in the system. References to contacts controlled by a specific
coil can be used as many times as needed to complete the control opera-
tion. Output coils that are not used to drive a specific load can be used
internally in the programming procedure.