Keywords: Homemade serial port optical isolator
Homemade serial optical isolator
In a system consisting of a serial port by multiple devices. It is often due to the distance between the equipment, the grounding is poor, or the environment is caused, causing a potential difference between the equipment, causing damage to the equipment. In this case, a serial optical isolator is used. It is an effective way to completely isolate between devices. A system of the author unit. It is a device consisting of a plurality of RS-232C serial interfaces. The device is far away. Every winter, the serial board of the main control equipment is often destroyed due to static electricity caused by climate drying. For this purpose, the author has a serial optical isolator to completely isolate the master device with other devices. Used, the phenomenon of the device serial board is destroyed throughout the winter.
I, circuit and principle
The electrical principle is shown in the drawings. Each serial port line uses a completely identical pair of isolation circuits, divided into two parts. Each part is made into a circuit board, respectively, on the serial port of the proximal end and the distal end. The two isolators are connected between 4 signal lines, T1 + even R1 +, T1-even R1-, similar, T2 + even R2 +. T2 - even R2-. On the other end of each isolator, data transmission (TD), data reception (RD), device preparation (DTR), and ground signal is required to be connected to the proximal and remote device serial port. In the circuit, "DTR" is supplied with the positive level for the photocoupler U1, and the capacitance C1 is the energy storage element; "TD" is supplied with the electrocoupler U2, like C1, and the capacitor C2 is Energy storage components. Further, "TD" is positive and negative to the input of the resistor R2 to the distal photocoupler U3, U4. The input positive and negative of the distal photocoupler U3, U4 is connected to the signal ground of the proximal device serial port, and the TD signal forms a closed loop. When the distal Td is high. The U1 turns on the U2 cut-off, the high level is added to the RD of the proximal device serial port, and the proximal reception signal is high. When the distal Td is low, the U2 is turned on, the lower end is added to the RD of the serial port of the proximal device, and the proximal received signal is low. In turn.
Second, the main component parameters
D1, D2, D3, D4 switch diode 1 N4148. Capacitors C1, C2, C3, C4 are used for 10 μF electrolysis. The withstand voltage is not less than 25V. All resistance uses L / 4W. According to the distance between the two devices and the requirements of anti-interference performance, and the current transmission ratio of the photocoupler used, R2 and R5 can be selected from different resistance. The smaller the resistance, the larger the transmission current, the stronger the anti-interference ability, but because the current can be limited (no more than 20mA), the resistance is not too small. If the resistance is too large, the anti-interference capacity of the line will decrease, and the transmission distance will also be affected, so select 1kΩ to 3kΩ is preferably, this circuit is 2kΩ. The resistance of the remaining resistance is selected for LKΩ, and the selection of the photocoupler mainly considers the current transmission ratio and the isolation voltage. The circuit uses 4N35, and its current transmission ratio is about 50%, the isolation voltage is 1500V.
Third, use precautions
After the two board is made, it can be loaded into the small plastic box. In order to ensure the isolation, the circuit board can be used to clean the circuit board. The corresponding pin of the various signals of the serial port should be checked before the access line. If the serial port does not provide a high level DTR signal, an external + 12V power supply is required. Author: Sun Haikan
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