LED display driver chip maintenance information

1. The main display screen driver IC 1.74HC04's function: 6-bit inverter. Pin 7 GND, power ground. Pin 14 VCC, power positive. The signal is input from the A end and output from the Y end in reverse phase. A1 and Y1 are a group, and the others are analogous. For example: A1 = "1", then Y1 = "0", A1 = "0", then Y1 = "1", and the functions of other groups are the same. The function of 74HC138: 8-bit binary to decimal decoder. Pin 8 GND, power ground. Pin 15 VCC, power positive. Pins 1 to 3 A, B, C, binary input pins. Pins 4 to 6 are controlled by chip select signals. Only when pins 4 and 5 are "0" and pin 6 is "1", will they be selected, and the output is controlled by A, B, C signals. Any other combination will not be selected, and Y0~Y7 outputs all "1". By controlling the select pins to cascade, it can be expanded to sixteen bits. For example: G2A=0, G2B=0, G1=1, A=1, B=0, C=0, then Y0 is "0" and Y1~Y7 are "1". The function of 74HC595: LED driver chip, 8-bit shift latch. Pin 8 GND, power ground. Pin 16 VCC, power positive pole. Pin 14 DATA, serial data input port, display data enters from here, must have the cooperation of clock signal to shift in. Pin 13 EN, enable port, when the pin is "1", QA~QH ports are all "1", when it is "0", the output of QA~QH is controlled by the input data. Pin 12 STB, latch port, after the input data is transferred to the register, only a latch signal can be provided to send the shifted data to QA~QH ports for output. Pin 11 CLK, clock port, each clock signal will shift one bit of data into the register. Pin 10 SCLR, reset port. As long as there is a reset signal, the data shifted into the register will be cleared. The display screen does not use this pin and is generally connected to VCC. Pin 9 DOUT, serial data output port, transfers data to the next one. Pins 15, 1~7, parallel output port, also known as drive output port, drive LED. Function of 4953: row driver tube, power tube. There are two CMOS tubes inside, pins 1 and 3 are VCC, pins 2 and 4 are control pins, pin 2 controls the output of pins 7 and 8, and pin 4 controls the output of pins 5 and 6. Only when pins 2 and 4 are "0", 7, 8, 5, and 6 will output, otherwise the output is in high impedance state. Function of TB62726: LED driver chip, 16-bit shift latch. Pin 1 GND, power ground. Pin 24 VCC, positive power supply Pin 2 DATA, serial data input Pin 3 CLK, clock input. Pin 4 STB, latch input. Pin 23 output current adjustment terminal, connected to resistor adjustment Pin 22 DOUT, serial data output Pin 21 EN, enable input Other functions are similar to 74HC595, except that TB62726 is a 16-bit shift latch with output current adjustment function, but no high level will appear on the parallel output port, only high impedance state and low level state. The 74HC595 parallel output port has high level and low level output. The pin function of TB62726 is the same as that of 5026, and the structure is similar. 2. Understanding of common signals of LED display screen CLK clock signal: shift pulse provided to the shift register, each pulse will cause data to shift in or out one bit. The data on the data port must be coordinated with the clock signal to transmit data normally, and the frequency of the data signal must be 1/2 times the frequency of the clock signal. In any case, when the clock signal is abnormal, the whole board will be displayed in a disorderly manner. STB latch signal: Send the data in the shift register to the latch, and display the data content by lighting up the LED through the drive circuit. However, since the drive circuit is controlled by the EN enable signal, the premise of its lighting must be that it is enabled to be in the on state. The latch signal also needs to be coordinated with the clock signal to display a complete image. In any case, when the latch signal is abnormal, the entire board will be displayed in a mess. EN enable signal: The entire screen brightness control signal is also used for display screen blanking. Just adjust its duty cycle to control the change of brightness. When the enable signal is abnormal, the entire screen will appear dark, dim or tailing. Data signal: Provides the data required to display the image. It must be coordinated with the clock signal to transmit the data to any display point. Generally, the red, green and blue data signals are separated in the display screen. If a data signal is short-circuited to the positive or negative pole, the corresponding color will appear fully bright or dark. When the data signal is suspended, the corresponding color display is uncertain.

ABCD row signal: It only exists when the dynamic scanning display is displayed. ABCD is actually a binary number, and A is the lowest bit. If the binary expression of ABCD signal control is the maximum range of 16 rows (1111), only AB signal is needed in 1/4 scanning, because the expression range of AB signal is 4 rows (11). When the row control signal is abnormal, there will be display dislocation, highlight or image overlap. 3. Common fault handling methods (tools: multimeter, soldering iron, blade, screwdriver, tweezers... etc.) 4. Maintenance method of LED electronic display screen The problem must be handled in the way of primary first and secondary later. The obvious and serious problems should be handled first, and the minor problems should be handled later. Short circuit should be the highest priority. 1. Resistance detection method, adjust the multimeter to the resistance range, detect the ground resistance value of a certain point on a normal circuit board, and then detect the same point on another identical circuit board to see if it is different from the normal resistance value. If it is different, the scope of the problem is determined. 2. Voltage detection method: adjust the multimeter to the voltage range, detect the voltage to ground of a certain point of the circuit suspected of having problems, and compare whether it is similar to the normal value, otherwise the scope of the problem is determined. 3. Short circuit detection method: adjust the multimeter to the short circuit detection range (some are diode voltage drop range or resistance range, generally with alarm function), detect whether there is a short circuit phenomenon, and solve it first after finding a short circuit, so as not to burn other devices. This method must be operated when the circuit is powered off to avoid damaging the meter. 4. Voltage drop detection method: adjust the multimeter to the diode voltage drop detection range, because all ICs are composed of many basic units, but they are miniaturized, so when a current passes through a certain pin, there will be a voltage drop on the pin. Generally, the voltage drop on the same pin of the same model of IC is similar. The voltage drop value on the pin is compared, and the circuit must be powered off. This method has certain limitations. For example, if the device being detected is high-resistance, it cannot be detected. 4. Processing steps for common problems of unit boards Unit board failure: A. The whole board is not lit 1. Check whether the power supply and signal line are connected. 2. Check whether the test card can identify the interface. If the red light of the test card flashes, it is not recognized. Check whether the light board and the test card have the same power ground, or the light board interface has a signal short circuit to the ground, which makes the interface unrecognizable. (Intelligent test card) 3. Check whether the 74HC245 has a short circuit, and whether the corresponding enable (EN) signal input and output pins on 245 are poorly soldered or short-circuited to other circuits. Note: Mainly check the power supply and enable (EN) signal. B. During the point-slant scanning, the regular interlaced lines are not lit and the display screen overlaps. 1. Check whether there is a disconnection, poor soldering or short circuit between the A, B, C, and D signal input ports and 245. 2. Check whether there is a disconnection, poor soldering or short circuit between the A, B, C, and D output ports corresponding to 245 and 138. 3. Check whether the A, B, C, and D signals are short-circuited or a signal is short-circuited to the ground. Note: Mainly check the ABCD line signals. C. When all lights are on, one or several rows are not lit. 1. Check whether the line between 138 and 4953 is open, poorly soldered or short-circuited. D. When scanning rows, two or several rows (usually multiples of 2, regular) are lit at the same time. 1. Check whether the A, B, C, and D signals are short-circuited. 2. Check whether the output terminal of 4953 is short-circuited with other output terminals. E. When all lights are on, one or more points (irregularly) are not lit. 1. Find the control pin corresponding to the module to measure whether it is short-circuited with the row. 2. Replace the module or single lamp. F. When all lights are on, one or several columns are not lit. 1. Find the pin that controls the column on the module and test whether it is connected to the output terminal of the driver IC (74HC595/TB62726). G. There is a single point or a single column that is highlighted, or the entire row is highlighted and is not controlled. 1. Check whether the column is short-circuited with the power ground. 2. Check whether the row is short-circuited with the positive pole of the power supply. 3. Replace its driver IC. H. The display is chaotic, but the signal output to the next board is normal. 1. Check whether the corresponding STB latch output terminal of 245 is connected to the latch terminal of the driver IC or the signal is short-circuited to other lines. I. The display is chaotic and the output is abnormal. 1. Check whether the clock CLK latch STB signal is short-circuited. 2. Check whether the clock CLK of 245 has input and output. 3. Check whether the clock signal is short-circuited to other lines. Note: Mainly check the clock and latch signals. J. Display is missing color. 1. Check whether the data terminal of the color of 245 has input and output. 2. Check whether the data signal of the color is short-circuited to other lines. 3. Check whether the cascaded data ports between the driver ICs of the color are open or short-circuited, or have poor soldering. Note: The voltage detection method can be used to find the problem more easily. Check whether the voltage of the data port is different from the normal one to determine the fault area.

K. Output problems 1. Check whether the line from the output interface to the signal output IC is connected or short-circuited. 2. Check whether the clock latch signal of the output port is normal. 3. Check whether the cascade output data port between the last driver IC is connected to the data port of the output interface or short-circuited. 4. Are the output signals short-circuited to each other or short-circuited to the ground? 5. Check whether the output wiring is in good condition. Whole screen failure: A. The whole screen is not lit (black screen) 1. Check whether the power supply is powered on. 2. Check whether the communication line is connected and whether it is connected incorrectly. (Synchronous screen) 3. The synchronous screen checks whether the green light of the sending card and the receiving card is flashing. 4. Whether the computer monitor is protected, or the display area of ​​the display is black or pure blue. (Synchronous screen) B. The whole unit board is not lit (black screen) 1. Several consecutive boards are not lit in the horizontal direction, check whether the wiring connection between the normal unit board and the abnormal unit board is connected; or whether the chip 245 is normal, 2. Several consecutive boards are not lit in the vertical direction, check whether the power supply of this column is normal. C. The uplink of the unit board is not lit. 1. Check whether the uplink pin is connected to the output pin of 4953. 2. Check whether 138 is normal. 3. Check whether 4953 is hot or burned. 4. Check whether 4953 has a high level. 5. Check whether 138 is connected to the control pin of 4953. D. The unit board is not lit. 1. Check whether 595 is normal. 2. Check whether the corresponding pins of the upper and lower modules are connected. 3. Check whether the output pin of 595 is connected to the module pin. E. The unit board is missing color. 1. Check whether 245R.G data is output. 2. Check whether the normal 595 output pin is connected to the abnormal 595 input pin.