Ladderdiagram (LD)

The ladder diagram is also a graphics oriented programming language which approaches the structure of an electric circuit.

On the one hand, the ladder diagram is suitable for constructing logical switches, on the other hand one can also create networks as in FBD. Therefore the LD is very useful for controlling the call of other POUs. More about this later. The ladder diagram consists of a series of networks. A network is limited on the left and right sides by a left and right vertical current line. In the middle is a circuit diagram made up of contacts, coils, and connecting lines.

Each network consists on the left side of a series of contacts which pass on from left to right the condition "ON" or "OFF" which correspond to the Boolean values TRUE and FALSE. To each contact belongs a Boolean variable. If this variable is TRUE, then the condition is passed from left to right along the connecting line. Otherwise the right connection receives the value OFF.

Example of a typical network in the ladder diagram as it could appear in TwinCAT PLC Control :

Contact

Each network in LD consists on the left side of a network of contacts (contacts are represented by two parallel lines: | |) which from left to right show the condition "On" or "Off". These conditions correspond to the Boolean values TRUE and FALSE. A Boolean variable belongs to each contact. If this variable is TRUE, then the condition is passed on by the connecting line from left to right, otherwise the right connection receives the value "Out". Contacts can be connected in parallel, then one of the parallel branches must transmit the value "On" so that the parallel branch transmits the value "On"; or the contacts are connected in series, then contacts must transmit the condition "On" so that the last contact transmits the "On" condition. This therefore corresponds to an electric parallel or series circuit. A contact can also be negated, recognizable by the slash in the contact symbol: |/|. Then the value of the line is transmitted if the variable is FALSE.

Coil

On the right side of a network in LD there can be any number of so-called coils which are represented by parentheses:( ). They can only be in parallel. A coil transmits the value of the connections from left to right and copies it in an appropriate Boolean variable. At the entry line the value ON (corresponds to the Boolean variable TRUE) or the value OFF (corresponding to FALSE) can be present. Contacts and coils can also be negated (in the example the contact SWITCH1 and the coil %QX3.0 is negated). If a coil is negated (recognizable by the slash in the coil symbol: (/)), then it copies the negated value in the appropriate Boolean variable. If a contact is negated, then it connects through only if the appropriate Boolean value is FALSE.

Function blocks in the ladder diagram

Along with contacts and coils you can also enter function blocks and programs. In the network they must have an input and an output with Boolean values and can be used at the same places as contacts, that is on the left side of the LD network

Set/Reset coils

Coils can also be defined as set or reset coils. One can recognize a set coil by the "S" in the coil symbol: (S)) It never writes over the value TRUE in the appropriate Boolean variable. That is, if the variable was once set at TRUE, then it remains so. One can recognize a reset coil by the "R" in the coil symbol: (R)) It never writes over the value FALSE in the appropriate Boolean variable: If the variable has been once set on FALSE, then it remains so.

LD as FBD

When working with LD it is very possible that you will want to use the result of the contact switch for controlling other POUs. On the one hand you can use the coils to put the result in a global variable which can then be used in another place. You can, however, also insert the possible call directly into your LD network. For this you introduce a POU with EN input. Such POUs are completely normal operands, functions, programs, or function blocks which have an additional input which is labeled with EN. The EN input is always of the BOOL type and its meaning is: The POU with EN input is evaluated when EN has the value TRUE. An EN POU is wired parallel to the coils, whereby the EN input is connected to the connecting line between the contacts and the coils. If the ON information is transmitted through this line, this POU will be evaluated completely normally. Starting from such an EN POU, you can create networks similar to FBD.

Part of a LD Network with an EN POU