How to read PLC ladder diagram

According to the control tasks needed to complete the control system, the control process, the control law, the function

 and the characteristic of the controlled process, the working characteristic and the control system are analyzed in detail,

 and the input and output physical quantity is the switch quantity or the analog quantity, Divide the control of the various 

stages and their characteristics, the transition between the conditions, draw a complete work flow chart and the

 implementation of the action of the components of the beat table.

Understand the input signal and the corresponding input relay configuration, the output relay configuration and the

 corresponding load. In the case of I / O configuration without input / output device definition and PLC, input / output device definition and I / O of PLC should be made according to PLC I / O wiring diagram or ladder diagram and instruction list Configuration.

1) According to the electrical circuit (such as motor, solenoid valve, electric heater, etc.) main circuit control electrical

 (contactor, relay) the main contact of the text symbols, in the PLC I / O wiring diagram to find the corresponding

 programming components of the coil , You can learn to control the control of the electrical output relay, and then in the

 ladder or statement table to find the output relay block, and make the mark and description. 〗 〖JP

2) According to the PLC I / O wiring diagram of the input device and its corresponding input relay, in the ladder 

(or statement table) to find the input relay contact, dynamic contact, and make the appropriate mark and Description.

Relay circuit diagram of the AC contactor and solenoid valves and other actuators with PLC output relay to control their 

coil connected to the PLC's output. Buttons, control switches, limit switches, proximity switches, etc. to provide PLC control

 commands and feedback signals, their contacts connected to the PLC input.

The functions of the intermediate relays and time relays in the relay circuit diagram are performed with auxiliary relays 

and timers inside the PLC, independent of the input and output relays of the PLC.

In the ladder diagram, if more than one coil is controlled by a contact string / parallel circuit, in order to simplify the

circuit, in the ladder diagram can be set with the circuit control of the auxiliary relay, auxiliary relay similar to the relay

 circuit in the intermediate relay The

Time relay in addition to delay the action of the contacts, there are in the coil when the power loss or immediate action 

instantaneous contact. For time relay with instantaneous contact, the auxiliary relay can be connected in parallel to the 

coil of the corresponding timer in the ladder diagram. The latter contact is equivalent to the instantaneous contact of the 

time relay.

In order to prevent the simultaneous control of the forward / reverse contact of the two contactors at the same time,

 resulting in three-phase power short-circuit, in addition to the ladder diagram with their corresponding output relay coil in 

series with the dynamic interlocking circuit composed of soft interlock, The hard interlock circuit should be set outside the 

PLC.

Whether to use the general programming method or the sequential function diagram programming method; using the

 single sequence structure of the sequential function diagram or selecting the sequence structure, the parallel sequence

 structure, using the start / stop circuit, the step sequence instruction or the set / reset Instruction to program. See Section

 4 and Chapter 5 for this section.

The decomposition of the ladder diagram by the operation of the main circuit (such as button) to start, check the line to

 track the main circuit control electrical (such as contactor) action, the middle to go through a lot of programming 

components and circuits, find it more difficult.

No matter how complex the ladder diagram, are composed of some basic units. According to the composition of the main

 circuit, the use of the inverse trace source method, the ladder and instruction statement table decomposition with the 

main circuit of the electrical (such as motor) corresponding to a few basic units, and then a link, a link analysis, and 

finally And then use the smooth track to keep the links together.

There are many trip switches and switch switches in the I / O wiring diagram of the PLC, as well as pressure relays,

 temperature relays, etc. These electrical components do not attract coils, and their contact actions are achieved by 

means of external forces or other factors. The external force or factors that cause the action of these contacts to be 

found. Wherein the stroke switch is touched or released by the mechanical linkage mechanism, and the changeover 

switch is normally operated by hand so that the contacts of the trip switch and the changeover switch are in different 

working conditions during the operation of the apparatus, i.e., the closing of the contacts , Disconnect the situation is

 different to meet the different control requirements, which is a key to see the process.

These trip switches, switches, contacts of the different working conditions alone to see the circuit diagram is difficult to

 figure out, must be combined with the device manual, electrical components schedules, clear the trip switch, the use of

 the switch, the operation of the switch switch mechanical linkage, touch Point in different closed or disconnected state of

 the circuit and so on.

(2) the use of reverse trace source method to multi-load (such as multi-motor circuit) into a single load (such as single 

motor) circuit

According to the main circuit to control the load control of the main electrical contact symbol, in the PLC I / O wiring 

diagram to find the control of the load contactor coil output relay, and then in the ladder and instruction statement to find out the control table The output relay coil and its associated circuit, which is to control the load of the local circuit.

In the ladder diagram and instruction statement table, it is easy to find the output relay coil circuit and its power, loss of 

power conditions, but caused the coil power, loss of power and its related circuit is not easy to find, Tracing method to 

find:

1) in the output relay coil circuit string, parallel to the other programming element contact closure, disconnect is the 

output relay power, loss of power conditions.

2) from these contacts to find their coil circuit and its related circuit, in these coil circuit will have other contactors, relay 

contacts ... ...

It is worth noting that when a programming element is energized or de-energized, it should be found that all of the front

 and rear stages of the programming element are driven by all the contacts of the programming element.

Find out a moving element in other circuits in the dynamic contact, dynamic contact, these contacts for other programming

 components of the power, loss of power to provide conditions or interlock, interlock to provide conditions, causing other 

electrical components action , Drive the implementation of electrical appliances.

The local circuit that controls the single load may still be complex and requires further decomposition until it decomposes 

into the basic unit circuit.

1) If the motor spindle connected to the speed relay, the motor according to the speed control principle of the composition 

of the parking brake circuit.

2) If the motor main circuit connected to the rectifier, indicating that the motor using energy consumption brake circuit.

The process of reading the PLC ladder and the statement table is the same as that of the PLC scanning user process, from

 left to right, top to bottom, according to the order of the block segment.

It is worth noting that during the execution of the program, in the same cycle, the preceding logical operation results in the

 subsequent contact, that is, the execution of the program used in the previous results of the latest intermediate operation.

 But in the same week, the subsequent logical operation results do not affect the previous logical relationship. The final status of all internal relays except the input relay (coil on or not, contact off or not) will affect the continuity of the contacts in the next scan cycle.

Since many readers are familiar with the relay contactor control circuit, it is recommended to follow the following steps to

 see the ladder diagram by following the read relay contactor control circuit check-

1) According to I / O equipment and PLC I / O allocation table and ladder diagram, find the input and output relays, and 

gives the relay contactor control circuit corresponding to the text code.

2) Mark the text of the corresponding input device and output device next to the ladder programming element coil and its 

contacts.

3) decompose the ladder diagram into a number of basic units, each of which may be a block of a ladder diagram 

(including an output element) or several blocks (including several output elements), each of which is equivalent to a relay

 Contactor control

5) a programming element is energized, all of its moving contacts are closed, moving contact are open. A programmed 

component is de-energized, and all closed contacts are disconnected (reset) and all disconnected dynamic contacts are 

closed (reset). So the programming element is power, loss of power, to find all of its dynamic contact, dynamic contact, 

analysis of its impact on the corresponding programming components.

6) In general, the ladder can be read from the first natural line of the first block. The first natural behavior program starts 

the line. Press the start button, connected to an input relay, the input relay all the moving contacts are closed, moving off 

contact are disconnected.

  And then find out the input relay which is closed by the input relay and the effect of breaking the contact off and 

analyzing the operation of these programming elements to determine the function of these programming elements. It is

 worth noting that some of these programming elements may be immediately electric action, and some do not immediately

 action but only for its electric power to prepare.

  According to the working principle of the PLC, when the input end contacts the contact, if the input contact is closed 

when the PLC is working, the input relay coil corresponding to the input terminal is energized and its moving contact 

closure , When the input contact is broken, when the input terminal is connected to the contact and the PLC is working,

 if the dynamic contact of the input terminal is not active, the input relay coil corresponding to the input terminal is 

energized and its movable contact Closed, moving contact open. If the dynamic contact is in series with the output relay coil, the output relay coil can not be energized. Therefore, with the PLC to control the motor start and stop, if the stop button with the dynamic contact, then with the control motor contactor connected to the PLC output relay coil should be connected with the stop button input contact pin corresponding to the contact The In the relay contact control, the stop button and the thermal relay both use the dynamic contact, in order to contact with the relay control control circuit in the PLC ladder diagram, also with the dynamic contact, so that, and The input end of the stop button and thermal relay contacts must be used to meet the contact. This must be taken into account when reading a program.

  When analyzing the functions of the PLC control system, it can be imagined as a control box in a relay control system.

 The external wiring diagram describes the external wiring of the control box. The ladder diagram or statement table is the

 internal " The input relay and output relay in the ladder diagram are the "interface relays" associated with the external 

world, so that the PLC control system can be analyzed by analyzing the relay circuit diagram. In the analysis can be ladder

 diagram of the input relay contacts as the corresponding external input device contacts or circuits, the output relay coil as

 the corresponding external load of the coil. External load of the coil in addition to the control by the ladder, but also may 

be controlled by external contacts