As the figure below shows, Y0 is set as OUT twice, which is referred to as double coil output In this case, it is the state
of X1 that actually drives external loads.
To resolve the confusion of double coil output, methods are recommended as follows:
Parallel connect the driving conditions and then output once.
Use the SET, RST instructions.
Use the CJ instruction.
Use the Step Ladder (STL) instruction or Sequential Function Chart (SFC)
A PLC usually performs the important part in the automatic control system. Appropriate protection of the PLC can
improve the reliability of the system.
Traditional PLCs which we are familiar with usually use AC power supply, relay output and terminal block wiring. The AC
power supply unit is susceptible to damage due to the instability of the power system (especially in non-industrial
advanced countries). The relay output has the lifespan limit because of mechanical contact. Also, to disassemble the
wiring at terminal blocks is time-consuming, inconvenient to fault repair (although has separable terminal products but
with higher price).
The best conguration for the new generation of PLCs should be the combination of DC power supply, transistor output
and connector wiring to exclude factors which affect the reliability from PLC systems as far as possible. In addition, this
way makes it possible to reduce wiring time, reduce costs and improve maintenance convenience. It would be more p
erfect if they are equipped with a memory card to store programs and data . When a PLC fails, the really important and
difcult part is to transfer the program (which need system-related professionals to deal with), rather than the work of
dissembling a terminal block (what a general electromechanical person can handle). With a memory card, the PLC
replacement work can become easy.
Before using a PLC, you should really understand the specications of each elements and avoid excessive use (with
particular attention to power capacity and output specications). Also, choose the right products according to your
needs in order to design a high cost-effective control system.
PLC is the core of a control system!
Having correct knowledge and ideas helps you establish a
stable and reliable control system.
˙˙˙˙˙˙˙˙˙˙
Y0
X0
X1
Y0
6
1-1-7 Double Coil Output
The PLC program with the following characteristic will affect the operation result, please pay special attention.
1. When executing the user program PLC follows sequential scan (from left to right, up to down).
2. In the PLC executing process, only the contents of the data memory will be used and changed. Actually to drive the
external loads are performed after the execution of the program and there is a procedure to output the computing
results.
1-1-8 Conclusion
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