Siemens 840D CNC System Screen garbled Code Fault Accurate Diagnosis Guide

Garbled text is a common fault on the 840D system screen, and over 90% can be repaired through systematic troubleshooting without the need to replace the entire machine. I will provide a complete and accurate diagnostic solution from four dimensions: fault classification, hardware detection, software repair, and preventive measures, to help you quickly locate and solve problems.

1、 Types of garbled characters and root cause classification

1. Global garbled code (chaotic characters on the entire screen)

Hardware issues (60%): unstable power supply, poor contact of LVDS interface, motherboard memory failure, CPU abnormality

Software issues (30%): System file corruption, missing Chinese character library, incompatible encoding

Interference problem (10%): electromagnetic interference, poor shielding

2. Local garbled code (abnormal display in some areas)

Cable problem: LCD screen cable oxidation or looseness

Display chip malfunction: Local damage to LCD controller

Font issue: Specific character set is missing (e.g. displayed as' □ 'in Chinese)

3. Special garbled characters (appearing in specific scenarios)

Alarm text garbled: mostly due to Chinese encoding issues or missing language packs

Program comment garbled: mostly due to incompatible encoding of text files (such as UTF-8 and ANSI conflicts)

2、 Hardware level diagnosis: from power supply to display link

Step 1: Power system inspection (primary troubleshooting)

checkpointstandard valueAbnormal judgmentinfluence

PCU20 input voltage24V DC±5%<22.8V or>25.2VSystem instability, abnormal display

The power board outputs 5V5V±0.25VFluctuations>5%Motherboard malfunction, data transmission error

Power board output 12V12V±0.6VFluctuations>5%Backlight or LVDS driver abnormality

Diagnostic method:

Measure the voltage of the PCU20 power terminal with a multimeter. If it is abnormal, check the external 24V power module (such as Siemens 6SN1145)

Open the chassis and check if the power board capacitor is bulging (a common problem with devices over 5 years old). Measure the output ripple and ensure it is less than 100mV

Key finding: Unstable power supply is the primary cause of garbled code faults (about 35%), often overlooked

Step 2: Display interface and cable detection (core step)

LVDS cable detection ('lifeline' displayed on 840D):

After the power is turned off, carefully unplug the LVDS cable (usually with a blue buckle) and observe if the gold finger is oxidized

Use a magnifying glass to check if there are any broken wires in the wiring harness (vulnerable at bends)

Re plug the cable (ensure the buckle is locked) and perform a power on test

Diagnostic points:

If the fault disappears after re plugging and unplugging, it is a problem of poor contact. It is recommended to use a conductive eraser to clean the golden finger

If the fault is alleviated but not eliminated, it may be due to a broken wire inside the cable, and the same type of cable needs to be replaced (cost about 50 yuan)

External monitor test: Connect the VGA cable to the VGA port of PCU20. If the display is normal, confirm that the screen or cable is faulty

Step 3: Diagnosis of motherboard memory and display chip

Memory detection (DDR2/DDR3 chips):

Open the PCU back cover and locate the memory module (usually 1-2 modules)

Pull out the memory module, clean the golden finger with an eraser, and reinsert it tightly

Upon startup observation, if the garbled code disappears, it indicates poor memory contact

If the fault persists, considering that the memory chip is damaged, professional tools (such as a memory tester) need to be used for testing

Motherboard capacitor inspection (a must-have skill with ten years of experience):

Focus on checking the electrolytic capacitors (1000 μ F/16V, etc.) around the CPU and Northbridge chip

If the top of the capacitor is bulging or leaking, replace it directly

Experience has shown that capacitor aging is the second leading hardware cause of garbled code in 840D systems (approximately 25%)

3、 Software level diagnosis: from system to display content

Step 1: Check system parameters and display settings

System language and display parameters:

Go to the system menu → 'Settings' →' Display ', check if the resolution matches the screen

Switch the system language (such as switching from Chinese to English and then back), and observe whether the garbled characters have changed

If only garbled characters are displayed in Chinese, it is likely a problem with the Chinese character library

PLC program inspection:

Check if the screen brightness is set incorrectly in the PLC (e.g. set to 0)

Confirm screen brightness parameters through HMI variable monitoring (usually named DBx. y)

Step 2: Chinese font and encoding repair (common software reasons)

Font file check:

Check if Chinese fonts (such as SimSun. htc) are missing from the system font directory (usually in C: \ \ Windows \ \ Fonts)

If the font file is lost, copy the corresponding font from the normal 840D system

Text encoding conversion (for program comments, alarm text garbled):

For text files imported via RS232/USB (such as alarm text), ensure to use ANSI or UTF-8 encoding

Avoid mixed encoding (some Chinese use GBK, some use UTF-8)

Open the text file with a multi encoding editor such as UltraEdit and save it again in a system compatible format

Step 3: System Files and Memory Cleanup

System memory optimization:

Close unnecessary background programs and free up system memory

Check the memory usage through the system diagnostic function (HMI menu → 'Diagnosis')

System file repair:

Connect the system using SinuCom NCU software and check the integrity of system files

If file damage is found, reload the system backup

If there is no backup, you can try to 'restore factory settings' (but parameters will be lost and need to be backed up in advance)

4、 The 'ultimate skill' summarized from ten years of maintenance experience

1. 'Three board axe' quick repair method

Power off and restart (simple yet able to solve 15% of temporary contact problems)

Turn off the main power, wait for 30 seconds (ensure the capacitor discharges), and then restart

Second Axe: Cleaning Technique (solving 30% of poor contact and oxidation problems)

Key cleaning: Dust and oil stains on LVDS cable interfaces, memory modules, and motherboards

Use electronic grade WUSHUI alcohol and compressed air tanks, avoid using regular water

Third Axe: Capacitor Revolution (for devices over 5 years old, immediate effect)

Directly replace all bulging or leaking capacitors on the power board and motherboard (cost approximately 100 yuan)

Priority should be given to replacing high-capacity electrolytic capacitors such as 1000 μ F/16V and 470 μ F/16V

2. Accurate positioning table for garbled code faults

Fault phenomenonPriority inspection itemstest methodrepair solution

Normal after garbled code at startupPower start timingOscilloscope measuring rising edge of power outputReplace the filtering capacitor on the power board

Randomly garbled code during operationMemory or LVDSMemory testing+re plugging of cablesClean the memory and replace the wiring harness

Only display garbled characters in ChineseFont/EncodingSwitch to English display testInstall Chinese font package+convert encoding

Fixed area garbled codeLCD cable/screenExternal monitor testingReplace the cable or LCD screen

Alarm text garbledAlarm file encodingView alarm files using UltraEditRe edit the alarm text to ANSI encoding

5、 Preventive measures (to avoid recurrence)

Regular maintenance: Clean the PCU interior once a quarter, with a focus on cleaning the fan and heat sink to prevent dust accumulation and overheating

Power protection:

Ensure that the system power supply is well grounded (to eliminate interference)

Consider installing UPS (to prevent voltage fluctuations)

Parameter backup:

Backup NC/PLC parameters and alarm texts to a USB drive once a month

Establish a 'garbled dedicated backup' (including Chinese fonts and correctly encoded text files)

Cable fixing:

Fix the LVDS cable with zip ties to reduce poor contact caused by vibration

Avoid excessive bending of cables (curvature radius>2cm)

6、 Comparison of Maintenance Costs (Ten Year Data)

Fault TypeComponent level maintenanceComponent replacementWhole machine replacement

Poor contact of the wiring harnessCleaning and re plugging (0 yuan)Replace the cable (50 yuan)3000-5000 yuan

Capacitor agingReplace capacitor (100 yuan)Replace the power board (500 yuan)5000-8000 yuan

Poor memory contactClean memory (0 yuan)Replace memory module (200 yuan)8000-12000 yuan

Chinese character library is missingCopy font file (0 yuan)Reinstall the system (500 yuan)10000+yuan

Summary: Over 90% of the screen garbled faults in the Siemens 840D system can be repaired through systematic troubleshooting, without the need for expensive replacement of the entire machine. Accurate diagnosis can be carried out in the order of 'power supply → wiring → memory → software', and 95% of garbled code problems can be solved within 2 hours. If encountering motherboard chip level faults (such as damage to the North Bridge), it is recommended to contact a professional repair company in Yangon (the cost is about 1/5 of replacing the entire machine), and do not directly replace expensive display units.