Ladder diagram relay diagram in the structure, element symbols and logic control functions are similar, but they also have many differences, ladder diagram programming has its own rules.
1) each logical line always begins with a bus left, then contact connection, finally ending in the coil or the right bus (bus could not draw on the right). Note: the bus must have contact with the coil and coil cannot have any contact with the right bus.
2) ladder diagram of contact can be arbitrary in series or in parallel, but the relay coils can only parallel not in series.
3) contact number is not restricted.
4) under normal circumstances, with one coil can only appear once in the ladder diagram. In the program, with one coil used two or more times, known as the “double coil output.“ “Dual coil output“, some PLC be treated as a syntax error, absolutely do not allow some PLC front output treated as invalid, only the last output force; and some of the PLC, contains a jump instruction or step–ladder diagram in the instruction allowed double coil output.
5) nonprogrammable ladder must be equivalent conversion into programmable ladder diagrams.
6) when there is a couple of series circuits in parallel, contacts number of loops in series should be placed at the top, as shown in Figure 5-2A. There are several parallel circuit when connected in series, parallel contact number of the loop should be placed on the left, as shown in Figure 5-2B. This program is simple, clear statement is less.
Figure 5-2 the ladder diagram
In addition, in the design of ladder diagram of the relay contacts when the best input device designed more for the normally open all right, prone. Recommended users as possible with the input device normally open contacts connected with the PLC input, if some signals can only be normally closed input, can be entered by pressing equipment is often designed and corresponding input relay contacts in the ladder diagram reverse (normally open to normally closed, normally closed to normally open)