Introduction
There are many auxiliary relays in the PLC, none of them can receive external input signals, nor can they directly drive external loads. They only serve to transmit signals inside the PLC, not with the PLC. The external connection is an internal status sign, which acts as an intermediate relay in the relay control system. Its normally open and normally closed contacts can be used infinitely during the internal programming of the PLC. The contact cannot drive an external load. Its coil is driven by the contacts of various soft elements in the PLC, which can make the logical structure of the program clearer. .
Classification
1. General-purpose auxiliary relay The general-purpose auxiliary relay of FX series PLC has no power-off protection function. In the FX series PLC, except for the input relay and output relay component numbers which adopt octal system, the component numbers of other programming components adopt decimal system.
If the power supply is suddenly interrupted while the PLC is running, the output relays and general auxiliary relays will all turn OFF. If the power is turned on again, except for the ones that are turned ON due to external input signals, the rest will remain OFF.
2. Battery backup/latching auxiliary relay Some control systems require memorizing the instantaneous state of power interruption, and reappearing its state after re-energizing. The battery backup/latching auxiliary relay can Used in this occasion. When the power supply is interrupted, use the lithium battery to maintain the contents of the shadow register in the RAM, or save them in the EEPROM. They just keep the state of the instantaneous power-off in the first scan cycle after the PLC is re-powered. In order to use their power-off memory function, a circuit with memory function can be used. Assuming that X0 and X1 in Figure 3-7 are the start button and the stop button, respectively, M500 controls the external motor through Y0. If the M500 is in the 1 state when the power is interrupted, because of the memory function of the circuit, the M500 will remain in the l state after the power is turned on again. Make Y0 continue to be ON, and the motor restarts to run.
3. Special auxiliary relays
Special auxiliary relays have a total of 256 points. They are used to indicate certain states of the PLC, provide clock pulses and signs (such as carry and borrow signs), and set the operation of the PLC Mode, or used for step sequence control, prohibit interruption, set the counter to count up or down, etc. Special auxiliary relays are divided into two categories:
(1) Contact utilization type
The coil of the contact utilization type special auxiliary relay is driven by the PLC system program, in the user program The contacts are used directly, but their coils cannot appear. Here are a few examples: M8000 (operation monitoring): When the PLC executes the user program, M8000 is ON; when the execution is stopped, M8000 is OFF (See Figure 3-8).
M8002 (initialization pulse): M8002 is ON only during one scan period when M8000 turns from OFF to ON (see Figure 3-8), and the normally open contact of M8002 can be used to make a break The components of the electric hold function are initialized and reset or set to their initial values.
M8011～M8014 are 10ms, 100ms, 1s and 1min clock pulses respectively.
M8005 (Lithium battery voltage drop): When the battery voltage drops to the specified value, it becomes ON, and its contacts can be used to drive the output relay and external indicator light to remind the staff to replace the lithium battery.
(2) Coil drive type
The coil is driven by the user program to make the PLC perform specific operations, and the user does not use their contacts. For example:
After the M8030 coil is "powered on", the "battery voltage drops" LED goes out;
When the M8033 coil is "powered on", after the PLC enters the STOP state, all output relays The status of M8034 remains unchanged;
When the coil of M8034 is "powered on", all outputs are prohibited;
When the coil of M8039 is "powered on", the PLC will work at the scan time specified in D8039 .