1. Problem:
1. HMI of Allen-Bradley PLC with model # 6181P-122TSXP was not showing display. This HMI is used in centrifuge and costs around INR 7 Lakhs.
2. Tools required:-
1. Miniature screw driver set 2. Soldering station 3. Oscilloscope 4. PC motherboard diagnostic card 5. Multi-meter 6. mobile phone camera with 10X magnification
.
3. Troubleshooting Steps:-
3.1.Inserted a 4 digit PC motherboard diagnostic card in the pci slot and powered up the machine, the numbers on digit display in card were changing and these numbers correspond to errors on mainboard. for ex, if the numbers stopped at 01 , implies the cpu has gone faulty or 2C implies bad memory etc. However, the number did not stop at these stages . Therefore, it was concluded that boot process was normal and problem was in LCD display. The video shows how it is done.
3.2 The LCD/LED display has two connectors coming from the mainboard , one is the 20 pin LVDS(Low Voltage Differential signalling) signal carrying the data to be displayed on LCD and other the power. The LVDS stands for low voltage differential signalling, and has three pairs differential signal and one pair of clock signals. The LVDS signals were verified with oscilloscope, and positive (up going signal in oscilloscope) and negative (down going signal in oscilloscope) was seen in the respective + and - pins in each differential pairs. Also, the voltage on these pins were tested with multi-meter to give about 1.3 V. The clock signal yielded a steady 4 MHz clock.
3.3. Thus, the problem appears to be that of back-light in LED display. The model of the display used is G12S1-L02. The datasheet of the display was also found on net and shared here. The back-light is provided by a string of LEDs which illuminates the LCD display. The LEDs require a voltage of about 20V to illuminate. The LED controller boosts the input voltage of 10-12 V to this voltage. Click here to see repair in progress...
3.4 The display was opened, and indeed, the back-light was not glowing. The G121S1-L02 LCD display has two PCBs one is LCD2MB pcb which gets the power from the mainboard and supplies the +12V power, EN (Enable) signal to the E88441 PCB which generates the actual timing, control and back-light voltage for driving the LCD and back-light LEDs. The PCBs are shown in figure 4 . The LED display driver in E88441 PCB was identified by using 10X magnification on mobile camera and the part number noted down. Datasheet of this IC was downloaded.
3.5. On studying the datasheet _A8501 it was found that that pin # 28 of this IC had to be supplied with EN signal, which is a 5 v PWM (Pulse Width Modulation) signal, which can also control the brightness depending on PWM waveform width. The EN signal was being generated by the LCD2MB card in the display panel. There is a touch point (TP) to verify the presence of this signal, which yielded no value with multi-meter, so, this signal did not reach the back-light driver and failure was traced to faulty resistors (R94,R82 & R104) on board. This figure-5 shows the A8501 IC magnified 10X by mobile-phone camera.
3.6. The circuit in and around the LED driver IC U1 was traced with the help of multi-meter continuity testing and 10x magnification in mobile phone camera and all the required transistors and MOSFET identified and re-drawn on paper to understand the working. The circuit with some explanatory notes is given here-fig6
3.7. It appeared that the resistors which were damaged were part of a 'power saving' mechanism to give reduced supply voltage of 10.73 V instead of 12 V to the display driver IC in the absence of EN signal. This mechanism was built around transistor Q7 and Q14 and MOSFET Q15 . These resistors are 1% tolerance SMD resistors following the EIA-96 pattern of naming. The resistors which were damaged were marked as 70D, and 10S which have the value of 536K and 12.4 ohm respectively.
3.8. These resistors could not be sourced so a workaround was found to get the EN signal directly to IC U1 by shorting a path in PCB and bypassing the 'power save' circuitry by running the driver IC at 10.73 volts as is evident by seeing the circuit diagram traced above. The repair is shown in this figure-7. The resulting voltage at LED back-light end was 20 V, which was sufficient to drive the back-light LEDs.
3.9. Assembled everything back, the display working ok. figure-8 & figure-9
4. Annexure:-
4.1 For Full HD TV, 30 pin (10 pairs) connector is used. For HDTV, 20 pin (5 pairs--> 4 data pairs & 1 clock pair , 4 data pairs give 8 bits) connector is sufficient. These 4 data pairs correspond to R,G,B signal and B+Hsync and V signal. For this particular 12 inch display 3 data pairs (Rin 0,1 & 2 corresponding to R,G,B) and one clock pair is used , the unused 1data pair(Rin3)(B+Hsync and V) the + is connected to ground and - is NC. Therefore, 3 data pairs contribute 6 bits 20 pin lvds cable with pinout .more number of pairs means more bits can be shown implying higher resolution.
4.2 Youtube video about LVDS
4.3 Some of the ICs identified in mainboard (DS90C385A-> LVDS serializer, Chrontel CH7308B--> PLL transmitter, ISL6262CRZ--> 2 phase buck converter)
4.4 LCD power connector from mainboard to LCD2MB board
5. Key Learnings:
5.1. Details of backlight LED driver IC and its working
5.2. LVDS signals and 4/6/8 bit differential signalling
5.3. EiA -96 1% resistor naming convention