The ladder diagram used has been explained using relays.  Figure 7-1 shows a block diagram of the overall system using relays.  It is sometimes called a HARD-WIRED control system.  When the control logic is installed it can only be changed manually.
NOTE:       The relay system is still preferred in some safety systems as it is very difficult to check the software operation of a PLC for faults.

A PLC collects inputs and distributes outputs in the same way as a relay logic circuit.  However, the relays are replaced by a microprocessor which is programmed to provide the switching logic. 
Figure 7-2 shows a typical PLC block diagram based on Allen-Bradley.

The Allen Bradley System 

Figure 8-1 shows the major components of an Allen-Bradley PLC.  It consists of the following units.

Main Processor Unit

This units provides the following functions
The system mP and RAM.
EEPROM memory compartment.  An EEPROM is normally added as a back-up to hold the RAM programme in case of system failure.
Input conditioning for 10 inputs with status indicators.
6 separate outputs with status indicators.
Battery compartment with lithium battery.  This battery supplies D.C. power to hold the RAM data if the mains supply is lost.
Communication port so that the processor RAM can be re-programmed.
The unit is powered by a standard single phase supply (UAE: 240V~50 Hz).  This supply is connected to the incoming line terminals.
Expansion I/O Unit
This unit is connected to the main processor unit by a cable.  It uses the connections shown to provide increased I/O's.  This provides an extra 10 inputs and 6 outputs.  There are status indicators for each I/O.  This unit gets d.c. power from the main processor unit.  A d.c. power indicator is provided to show that this unit is powered correctly.

Pocket Programmer

This unit has a keyboard and display panel.  It is used to programme (configure) the required logic operations.  This will be used in the workshop when you try some simple programming techniques.
The Allen-Bradley system described is one of the simple single mP types.  It is used for
                  Controlling a single process (e.g. pump starting, ship loading sequences etc.)
              Larger processes which have no effect on plant safety.  (Therefore they must be cheap, e.g. fire detector systems for accommodation units etc.)
         As a back-up for a large safety system.  (E.g. to operate a shut-down if the emergency shut-down button is pressed or a fire alarm if the "break glass" unit is operated).

NOTE:        Allen-Bradley also supplies a hard-wired relay expansion unit.  This unit is used if higher current switching is required.  Maximum 2.5A continuous when switching either 240V a.c. or 24V d.c.


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