Wiring Problems of Weighing Instruments And Anti-interference Design of Lines

Wiring problems of weighing instruments and anti-interference design of lines

1. Setting of communication parameters of weighing instrument

1) Communication mode

The weighing instrument and computer adopt RS232 serial port communication. If there is a given weighing instrument in the software system, it shall be selected directly. If not, it shall be set by itself. The weighing instrument shall be set in a continuous communication mode to always send data to the computer during operation.

2) Communication baud rate

The baud rate of the weighing instrument is generally 120024004800960019200, which is usually set to 4800.

2. Communication format parameters

The communication format of weighing instruments from different manufacturers is different, and the communication format of each weighing instrument is fixed. Only the communication format of the computer can be set to meet the needs of the weighing instrument. When setting the computer communication format, the communication format shall be determined according to the requirements of the selected weighing instrument manual.

The parameters of communication format include: start bit, data bit, stop bit, parity bit, etc. To determine the character of the start bit and the number of data bits, it is also necessary to determine whether the character adopts ASCII code or BCD code, and whether it is sent in positive order or reverse order. Some weighing instruments have many invalid bits, which should be shielded during setting. Only when each parameter is correctly selected can the normal communication be ensured.

For example, a weighing instrument

3. Connect the communication line

1) The connection between the output lead of the weighing instrument communication interface and the computer must be accurate. If the connection is wrong, the weighing instrument output port or computer communication input port may be damaged, and even the weighing instrument, computer and corresponding external equipment may be seriously damaged.

2) Before unplugging the communication connector, turn off the power first and then unplug it, otherwise the communication port will be damaged.

3) Computer communication must be participated in or guided by professional technicians. Non professionals shall not connect at will.

Anti interference design of weighing instrument wiring

The circuit part of the weighing instrument mainly adopts the assembly form of printed circuit board (PCB), and the level of PCB design directly affects various performance indexes of the product. The following describes the principles and methods of applying anti-interference theory to the anti-interference design of PCB of weighing instrument in practical work from the aspects of power supply part, analog part and digital part of PCB of weighing instrument

（1） PCB anti-interference design

From the L: working principle of the weighing instrument, it can be seen that the PCB design of the weighing instrument mainly includes the design of analog part, AC / DC power supply part, microprocessor digital part and display key part. The power filter and transformer are large and generally not installed on the circuit board.

L power supply design

The AC / DC power supply of the weighing instrument shall include the analog power supply, which is generally DC12 V. This part of the power supply has high stability requirements and should be far away from the high-voltage part and AC power supply part on the board; the other part is multi part power supply and display part power supply. Because the power supply part is relatively human and the thermal address is relatively concentrated, Considering the heat dissipation and its impact on other circuit parts, the power supply part of the general weighing instrument is made into a separate circuit board. If the board is not made separately, the power supply part shall also be placed on one side of the whole PC B board, and the power supply of the simulation part shall be arranged close to the simulator. During specific wiring, the ground wire of the power supply part shall be arranged according to the principle of human area grounding and single point grounding, the lines shall be as thick and short as possible, and the vias shall be as few as possible.

2 design of simulation part

Because the analog part transmits analog signals, the analog signals are easy to be disturbed and make the weighing inaccurate or unstable. Therefore, the PCB design of the analog part is the focus of the PCB design of the weighing instrument. The following points are the experience summary in practical work for common reference.

(1) From the perspective of anti-interference, the analog part is made into a separate circuit board to reduce the interference of other circuits of the weighing instrument. The whole board is shielded with a metal shell to reduce the interference of external electromagnetic field.

(2) an analog ground wire (the ground of the analog part) is set between the signal lines output by the sensor to reduce the crosstalk between signals, and a polyester film filter capacitor is added between the two signal lines.

(3) the components of the whole simulation part are placed in a centralized manner and surrounded by large-area simulation.

(4) except that the disaster removing capacitor is set between the power supply and the ground of each integrated circuit, the disaster removing capacitor of Lu f is configured between the analog power supply and the analog ground, and between the analog ground and the digital ground. On the one hand, it is used as the energy storage element of the power supply, and on the other hand, it bypasses the high-frequency noise. Otherwise, one test of electric pulse train may fail.

(5) Connect the elastomer of the sensor (generally, the sensor manufacturer connects the elastomer with the shielding layer of the sensor signal line) with the analog ground.

3 digital part design

The digital part of the weighing instrument shall include microprocessor and various logic gates. The following problems should be paid attention to in the actual design:

(l) Reset circuit and clock circuit of microprocessor shall be close to microprocessor;

(2) minimize the common impedance of the power line and ground wire of the logic gate circuit device, that is, reduce the common part of the power line and ground wire and reduce its current loop.

(3) Separate the digital ground wire from the analog ground wire, and finally connect it at a single point.

4 Design of display keys

In this part of the circuit board design, various components shall be placed according to the specific size requirements, but because it is a man-machine interface part, attention shall be paid to waterproof design in this part of the design. All printed lines shall be kept away from the edge of the board as far as possible. In addition, it should be noted that the ground wire of this part, digital ground wire and analog ground wire shall follow the principle of one-point grounding; For example, in case of poor grounding on the display and key board, when the displayed value is within the peeling range, execute the "peeling" function, and the display cannot return to zero.

For the electronic truck scale and other weighing instruments that need lightning protection design, the poor grounding or non grounding of the instrument PCB will cause the components to be burned and scrapped, resulting in serious consequences. It should be noted that this part contains high voltage of more than 30 volts and transmits changing pulse signals. If it is close to the analog part, it may cause inaccurate lifting, so this part should be far away from the analog part.

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