Segments of Ladder Logic
The ladder logic required to control the application is saved in a logic segment in user memory. If you are work subroutines as part of the application, the logic of subroutine have to be located in a divide segment. The Modicon Micro PLCs offer you a design with two segments in it.
• Segment 1 is normally scheduled of ladder logic used to control the function is saved.
• Segment 2 is all logic of subroutine is saved. Subroutines logic is examined only when it is identified, either from an external event or from the ladder logic that triggers an interrupt. Consequently segment 2 is not resolved as element of the regular logic scan.
Networks of Ladder Logic
Every segment is created of a group of neighboring networks. Every network is a small, clearly defined diagram of ladder surrounded on the left by a rail of power and on the right by a rail that, by convention, is not presented. The ladder is 7 rungs high by 11 columns wide. The junction of every column and rung in the network is defined a node—each network consists 77 nodes. There is no arranged limit on the networks number that can be placed a segment—overall size of program is restricted by the amount of program memory of user existing in the CPU and by the time it gets for the CPU to examine the program of ladder logic.
Placing Instruction and Relay Logic s in a Network
Every time you utilize a relay logic element—e.g. a horizontal short, a contact, a coil,—in ladder logic, the part uses one node in the logic network. A command in ladder logic may use one, two, or three nodes in one network, based on the type of instruction. A counter instruction, for instance, the two-high nodal instruction—it uses two neighboring nodes that have to be one over the other. An instruction of ADD, on the other hand, is an instruction of three-high nodal using three neighboring nodes loaded over each other.
A Modicon Micro PLC examines the program of ladder logic serially in the following order:
• Segment by segment
• n sequentially network 1 through network within every segment
• Node by node within every network, beginning in the ladder of upper left node and shifting top to bottom, followed by left to right