(1) shall comply with heat in the ladder diagram language syntax
Relay diagram, for example, contacts can be placed online on the left, or you can put the right side of the coil, but in a ladder diagram, coils and output class instruction (such as SET and application instructions and RST) must be placed in the circuit of the far right.
(2) intermediate unit
In a ladder diagram, if more than one coil is controlled by a touch point of series-parallel circuit, in order to simplify the circuit can be set in the ladder diagram of auxiliary relay controlled by the circuit, as shown in Figure 1 in the M0 and M1, which is similar to the following relay in the circuit.
(3) separation of intertwined circuit
The relay in the circuit, in order to reduce the use of the devices and the use of contact, so as to save on hardware costs, each coil control circuits are often inter-related, intertwined. If you convert a figure 2 directly without any changes of ladder diagram, you want to use a large number of push (MPS), MRD (MRD) and pop (MPP) command, transformation and analysis of these circuits are more troublesome. To separate the coil control circuit design (see Figure 1), it may use some contact, because there is no stack instructions, compared with the direct conversion method, used by the directive will not be much difference. Even more directive, will not increase the cost of hardware, the operation will not have any effect on the system.
Design of ladder diagram with coil for the unit, separate consideration of each coil is available in the following circuit diagram contact control circuit, and then draw the equivalent ladder circuits.
(1) normally closed contact provides the input signal processing
Design input circuit, should be used for normally open contacts, if I can use a normally closed contact, ladder diagram correspond to normally open/normally closed type contacts and relay diagram opposite. Figure 3 limit switch in the input circuit of the PLC SQI normally closed contacts are connected to the X4 Terminal, the normally closed contact of the relay diagram SQI SQI of normally closed contacts in the ladder diagram in the ladder diagram corresponds to the X4 normally open contact
(2) optimal design of ladder diagram circuit
In order to reduce the number of statement form instructions instruction, in a series circuit, a single contact point should be placed in the circuit to the right, in a parallel circuit, a single point of contact should be placed in the circuit below.
(3) the instantaneous contact point of time relay
Contacts in addition to deferred action, a time relay coil power on or power off action instantaneous contact immediately. Time relays for instantaneous contacts, can correspond in the ladder diagram timer coil in parallel auxiliary relays, which contact corresponds to the time of the instantaneous contact point of the relay.
(4) power off delay time relay
Figure 2 coil KT which come in time after a power failure delayed the start of the relay. FX series PLC timers do not have the same functionality, but it can be implemented with coil power on delay timer off time delay function (see Figure 1 in the following two circuits and waveforms).
(5) the establishment of the external interlock circuit
In order to prevent control of the pros and cons of two contacts at the same time, resulting in a three-phase power circuit should be set outside of the PLC hardware interlock circuit. In Figure 2, KM2 and KM3 KM4 KM5 coil cannot be powered on, respectively, except in the ladder diagram in the set and their corresponding output relay normally closed contact of coils in series software consisting of interlocking circuits should also be set outside of the PLC hardware interlock circuit.
(6) overload thermal relay signal processing
Under automatic reset thermal relay, the overload signal must pass through the input circuit contact provided available to the PLC (see Figure 3 for FR2), ladder diagram for overload protection. If belongs to the manual reset thermal relay normally closed contact can be in the output circuit of the PLC in the series with the coil of AC contactor control motor.
(7) minimize the PLC input signal and the output signal
PLC I/O points and price of, and reduce the number of input/output signal is the main measures to reduce hardware costs.
Generally only require the same input devices of a normally open or normally closed contact provides input to the PLC signal, in a ladder diagram, many times using the same input relay normally open and normally closed contacts.
Relay diagram, if several receiver contact points of series-parallel circuit only once or always as a whole appear more than once, they can be used as an input signal of PLC, accounted for one of the entry points of PLC.
Some contacts in the relay diagram, if only once, and in series with the load at the output of the PLC (if manual reset thermal relay normally closed contact), they don’t have to be as a PLC input signal, you can place them outside of the PLC output circuit, still in series with the external load.
Relay control systems in certain relatively isolated and relatively simple parts, you can use the relay circuit control, while reducing the PLC input and output points required.
For example figure 2 control circuit of the AC spindle motor contactor KM1 is quite simple, it has no contact with the other circuit, circuits like this there is no need to use PLC control, should remain controlled by a relay circuit.
(8) the rated voltage of the external load
The relay outputs of the PLC module and TRIAC output module can only drive the rated voltage AC 220V, or set outside of the PLC auxiliary relay.
PLC ONE 