ENTERING A PROGRAM
The manner in which a program is entered into the memory of the PLC is specific to the manufacturer and type of programming terminal used. Some programming terminals employ keys that contain con- tact, coil, and rung symbols to basically draw the program as it is entered. Small programming terminals may require that the program be entered in a language called Boolean. Boolean uses statements such as and, or, not, and out to enter programs. Contacts connected in series, for example, would be joined by and statements, and contacts that are connected in parallel with each other would be programmed with or statements. In order to program a contact normally closed instead of normally open, the not statement is used. Different PLCs also require the use of different numbers to identify particular types of coils. One manufacturer may use any number between 600 and 699 to identify coils that are used as timer and counters. Another manufacturer may use any number between 900 and 999 to identify coils that can be used as timers and counters. When programming a PLC, it is always necessary to first become familiar with the programming requirements of the model and manufacturer of the programmable logic controller being programmed.