What Is an Auxiliary Relay?

Auxiliary relays are implemented in software. They cannot receive external input signals or directly drive external loads. They are an internal status indicator and are equivalent to intermediate relays in a relay control system.

1. General auxiliary relay The general auxiliary relay of FX series PLC has no power-off protection function. In the FX series PLC, except for the component numbers of the input relay and output relay, which are in octal, the component numbers of other programming components are in decimal.

If the power is suddenly interrupted while the PLC is running, all output relays and general auxiliary relays will be turned OFF. If the power is turned on again, except for the external input signal, it will remain ON.

Power-off protection diagram

2. Battery backup / latch auxiliary relays Some control systems require memorizing the momentary state of power interruption, and reproduce the state after power-on again. Battery backup / latch auxiliary relays can be used in this case. Use a lithium battery to hold the contents of the shadow registers in RAM when power is interrupted, or save them in EEPROM. They only remain powered off for the first scan cycle after the PLC is powered on again. In order to take advantage of their power-off memory function, a circuit with a memory function can be used. Assume that X0 and X1 in Figure 3-7 are the start button and stop button, respectively. M500 controls the external motor through Y0. If the M500 is in the 1 state when the power is interrupted, the M500 will remain in the 1 state after the power is turned back on. Leave Y0 ON, and the motor will restart.

3 Special auxiliary relay

Special auxiliary relays have a total of 256 points. They are used to indicate some states of the PLC, provide clock pulses and flags (such as carry and borrow flags), set the PLC operation mode, or be used for step sequence control, disable interrupts, Determine whether the counter counts up or down. Special auxiliary relays fall into two categories:

(1) Contact utilization type

The system program of the PLC is used to drive the coils of the special auxiliary relay of the contact utilization type, and the contacts are used directly in the user program, but their coils cannot appear. Here are a few examples:

M8000 (operation monitoring): When the PLC executes the user program, M8000 is ON; when it stops executing, M8000 is OFF (see Figure 3-8).

Waveform

M8002 (initialization pulse): M8002 is ON only during one scan period when M8000 changes from OFF to ON (see Figure 3-8). The normally open contact of M8002 can be used to initialize the components with power-off hold function. Or set them to initial values.

M8011 M8014 are 10ms, 100ms, 1s and 1min clock pulses.

M8005 (Lithium battery voltage reduction): When the battery voltage drops to the specified value, it turns ON. You can use its contacts to drive the output relay and external indicator lights to remind the staff to replace the lithium battery.

(2) Coil driven type

The user program drives its coils and causes the PLC to perform specific operations. The user does not use their contacts. E.g:

After the coil of M8030 is powered on, the battery voltage decreases light-emitting diode goes out;

When the coil of M8033 is powered on, after the PLC enters the STOP state, the states of all output relays remain unchanged;

When the coil of M8034 is powered on, all outputs are disabled;

When the coil of M8039 is powered on, the PLC works with the scan time specified in D8039.