PROGRAMMING LOGIC CONTROLLERS (PLC)

There are many types of PLC software used in the world, such as Siemens, Hitachi, Mitsubishi, etc…In the CNC systems that we are retrofit, used siemens PLC software. But I learn write the PLC program and test it using Hitachi software. In the Hitachi PLC unit has input/output cards and device to run the program. There are 16 slots in the one card which means, we can in/out 16 signals by using one card. But it is vary according to the PLC unit that we are used. Each slot has individual address number. If it is an input card, the address number is begins at zero and varies as follows according to the card number

• 0 to 15 1st card

• 20 to 35 2nd card

• 40 to 55 3rd card...

If it is an output card, the address number begins at 200 and varies as follows according to the card number.


• 200 to 215 1st card
• 220 to 235 2nd card
• 240 to 255 3rd card...


1 PLC Programming languages

There are three types of PLC languages which mainly used for programming.

• Ladder diagram
• Instruction list
• Function block diagram

But normally, engineers are using ladder diagram for PLC programming. Because it is
very easy to construct the logic, and trouble shooting. There are four basic symbols
used to programming by using ladder diagram.

• Normally open (N/O) input
• Normally close (N/C) input
• Normally open (N/O) output
• Normally close (N/C) output

By using these basic symbols, we can build some basic logics which are used in the PLC
programming