The ladder diagram language is a symbolic instruction set that is used to create a programmable controller program. Before the extension of the ladder language, the standard ladder instruction set was limited to performing only relay equivalent functions, using the basic relay-type contact and coil symbols similar to those shown in figure 1. A necessary for greater flexibility, coupled with developments in technology, it is now extended to six sub-instruction sets and they are relay-type, timer/counter, data manipulation, arithmetic, data transfer, and program control. Desired control logic can be obtained by formatting the ladder instruction symbols and store into memory.
The aim of ladder diagram program is to control outputs based on input conditions. Ladder rung is used for the control . Control rung, in general, consists of a set of input conditions represented by relay contact-type instructions and an output instruction at the end of the rung represented by the coil symbol. A basic structure of the ladder rung is shown in figure 1. Coils and contacts are the basic symbols of the ladder diagram instruction set. The coil symbols represent all the outputs, where contacts represent conditions to be evaluated in order to determine the control of the output. Each coil and contact is referenced with an address number that is used to identify the operating conditions. i.e what is being evaluated and what is being controlled.
The format of the rung contacts is dependent on the desired control
logic. Contacts may be placed in any configuration such as series,
parallel, or series/parallel that is required to control a given
output. For an output to be activated or energised, at least one
left-to-right path of contacts must be closed. A complete close
path is referenced to as having logic continuity. When logic continuity
exists in at least one path, it is said that the rung condition
is TRUE. The rung condition is FALSE if no path has continuity.
Relay Type Instructions
