1. HMI of Allen-Bradley 6181P-122TSXP
Fig 1: The HMI make & Model
was not showing any display when powered up. This HMI is used in Centrifuge. The cost of this machine is around Rs Seven Lakhs.
2. 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 and continued up-to full boot. Therefore, it was concluded that boot process was normal and problem was in LCD display. The video
shows how it is done.
3. The LCD/LED display has two connectors coming from the mainboard , one is the 20 pin LVDS signal and other the power.
Fig 2: The Motherboard showing power & LVDS connectors
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. The clock signal yielded a steady 4 MHz clock.
4. Thus, the problem appears to be that of backlight in LED display. The model of the display used is G12S1-L02.
Fig 3: The LCD display panel G121S1-L02 showing the LCD2MB PCB
The datasheet of the display was also found on net and shared here. The backlight is provided by a string of LEDs which illuminates the LCD display. The power connector pin-out is shown here
5. To access the backlight string of LEDs, the display was opened , and indeed, the backlight was not glowing. The LCD display has two PCBs one is LCD2MB pcb (shown in fig 3) which gets the power from the mainboard and supplies the +12V power,EN signal to the E88441 PCB which generates the actual timing,control and backlight voltage for driving the LCD and backlight LEDs.
Fig 4: The LCD display opened revealing E88441 PCB containing the backlight controller
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. and can be viewed here
6. On studying the datasheet it was found that that pin # 28 of this IC had to be supplied with EN signal, which is a 5 v PWM signal, which can also control the brightness depending on PWM on time. 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 multimeter, so, this signal did not reach the display driver and failure was traced to faulty resistors (R94,R82 & R104) on board.
Fig 5: A8501 magnified 10x
7. The circuit in and around the LED driver IC U1 was traced with the help of multimeter continuity testing and all the required transistors and MOSFET identified and re-drawn on paper to understand the working. The circuit is given <here>
Fig 6: The backlight driver power save mechanism redrawn
8. 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 reistors which were damaged were marked as 70D, and 10S which have the value of 536K and 12.4 ohm respectively.
9. 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' circuitary by running the driver IC at 10.73 volts as is evident by seeing the circuit diagram traced above. The repair is shown in figure 7. The resulting voltage at LED backlight end was 20 V, which was sufficient to drive the backlight LEDs.
Fig 7: The soldered portion to supply EN signal directly to backlight controller
10. Assembled everything back, the display working ok.
Fig 8: HMI giving output after the repair
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. The three data pairs correspond to R,G,B signal and the fourth data pair is for B+Hsync and V. 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 1 data pair (Rin3 for B+Hsync & V) the + is connected to ground and - is NC. Therefore, 3 data pairs contribute 6 bits . The 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)
Fig 9 : 20 pin LVDS connector pin-out
Key Learnings:
1. Details of backlight LED driver IC and its working
2. LVDS signals and 4/6/8 bit differential signalling
3. EiA -96 1% resistor naming convention
