MITSUBISHI A-series PLC Troubleshooting Replacement

It is strictly prohibited to parallel the outputs of multiple power modules to the same I/O module for power supply, otherwise it may cause module heating or even fire.

2. Input module wiring

For DC input modules, the common terminal can be connected to the positive or negative pole to achieve source or drain type. For example, when the COM of A1SX40-S1 is connected to 0V, the input terminal is connected to 24V effectively (source type).

To improve anti-interference, the input signal line should be laid separately from the power line, with a spacing of ≥ 100mm. If necessary, shielded cables should be used, and the shielding layer should be grounded at one end.

For the communication input module (A1SX10EU), attention should be paid to the power supply frequency of 50/60Hz, input current of 6mA, and minimum ON voltage of 80VAC.

3. Wiring of output module

The common terminal (COM) of the relay output module can be connected to AC or DC power supply, but it must be consistent with the power supply of the load. Inductive loads must be connected in parallel with RC absorption circuits (AC) or freewheeling diodes (DC).

The "external power" terminal of the transistor output module must be connected to 24VDC, otherwise the output cannot conduct. The manual clearly requires' SELV power required '.

When driving incandescent lamps or capacitive loads, the impulse current may reach 10 times the rated value and needs to be reduced for use.

4. Communication wiring (A1SCPUC24-R2)

The RS-232C interface adopts a 9-pin D-sub male connector, with pin definitions of 2-RD, 3-SD, 5-SG, 7-RS, and 8-CS. Cable length ≤ 15m, it is recommended to use 7/0.127mm shielded twisted pair.

If the external device cannot provide the CD signal (pin 1), the RS232C CD terminal with buffer address 10BH needs to be checked as "not executed" in the program.

Self loop test: Short circuit 2-3 and 7-8 to verify if the interface hardware is functioning properly.

Fault safety circuit design

According to Chapter 5 of the manual, a safety circuit must be constructed outside the PLC to prevent danger caused by power failure or PLC hardware failure.

1. Essential external circuits

Emergency stop circuit (hard wired, not through PLC).

Positive and reverse interlock (contactor electrical interlock).

Hardware travel switch for upper/lower limit positioning control.

Output module monitoring circuit: For a single output point that may cause serious accidents (such as a motor contactor), an external relay should be connected in parallel, and its contacts should be connected to the PLC input for program monitoring of its status.

2. Power sequence control

To avoid accidental output during power on, the following sequence is recommended:

First, power the PLC.

Provide external load power supply (such as the load power supply of 24V output module).

After a delay of 0.5 seconds through the PLC program, connect the main contactor (MC) and allow the output action.

The specific example circuit is shown in Figure 5.2 (pages 5-2 of the manual). For the DC output module, voltage relay RA2 can be used to detect the establishment of the load power supply, and then a timer can be used to delay the connection of MC.

3. Safety status in case of PLC failure

When the PLC detects the following faults, all outputs will be turned off:

Power module overcurrent/overvoltage protection action.

CPU watchdog timer timeout (WDT ERROR).

Program execution error (such as instruction code error, no END instruction).

But for faults that the CPU cannot detect (such as I/O control bus damage), the output may randomly remain ON or OFF. At this point, it is necessary to rely on external safety circuits (such as the monitoring circuit or safety relay mentioned above) to cut off the hazard source.

Common error codes and troubleshooting

Based on the error code table in Chapter 7 of the manual, the most common errors and their handling methods on site are extracted.

1. Instruction code error (INSTRUCT CODE ERR., error code 10)

Phenomenon: CPU stops, ERROR LED stays on.

Possible reasons:

The memory contains instruction codes that the CPU cannot decode (such as damaged ROM or downloaded incompatible programs).

Noise causes changes in memory content.

Exclusion steps:

Read the wrong step with a programmer and check if the instruction for that step is valid.

If using an EPROM memory card, reprogram the correct program.

If it occurs frequently, check the grounding and power filtering.

2. Parameter Error (Error Code 11)

Phenomenon: CPU stops.

Possible reasons: Parameter settings exceed the CPU range (such as incorrect program capacity or file register capacity settings) or the total exceeds the memory card capacity.

Solution: Use peripheral devices to read parameters, reset and write them.

3. Missing END INS instruction (Error code 12)

Phenomenon: CPU stops.

Reason: There is no END (or FEND) instruction at the end of the program.

Solution: Write the END instruction at the end of the program.

4. Watchdog timer error (WDT ERROR, error code 22)

Phenomenon: CPU stops or ERROR flashes during RUN.

Reason:

The scanning time of the user program exceeds the set watchdog time (default 200ms).