Why I used this circuit to isolate the communication between the main control and the serial screen, and eliminate the interference of the mainboard radio frequency to the serial screen, but the result was...

QWE4562009

Why I used this circuit to isolate the communication between the main control and the serial screen, and eliminate the interference of the mainboard radio frequency to the serial screen, but the result was... [Copy link]

 

Serial communication optocoupler isolation circuit

Why I use this circuit to isolate the communication between the main control and the serial screen, and eliminate the interference of the mainboard radio frequency to the serial screen, but the serial port fails to communicate. When I turn on the computer, the serial screen cannot receive any instructions from the mainboard MCU, let alone touch setting parameters. What is wrong?

beyond_笑谈

Under high-frequency signals, PC817 must be replaced with a high-speed optocoupler, or a digital signal isolator, similar to the one shown below.

damiaa

In both figures, a level of npn transistor is added to the side with the diode so that the driving capability is sufficient and there is no reverse. In addition, the communication rate must be high, so a high-speed optocoupler is required.

If you don't want to add a transistor, you can first change the control (the end of the CPU's TX_S connected to the resistor and the end of the external input TX_P connected to the resistor) to be placed below the diode, and the power supply is connected above, and the resistor is smaller. This can also increase the driving ability, but whether it is enough still needs to be tested specifically. The pull-down input current (pull current) of the IO port is generally larger than the output current (sink current).

qwqwqw2088

No optocoupler required.

How much better it is to use digital isolator chip for isolation directly?

For example, ADuM1201, ISO7221

dukedz

You used this diagram, the data logic is reversed

se7ens

Same as the 5th floor suggestion

The data logic is reversed

The modification method can refer to the method on the 3rd floor. Connect tx and rx to the cathode of the optocoupler, and pull the anode of the optocoupler up to the power supply.

Or add an inverter to the input signal end

If it is too troublesome, you can use an isolator directly

QWE4562009

beyond_笑谈Published on 2024-7-10 21:04 Under high-frequency signals, PC817 must be replaced with a high-speed optocoupler, or a digital signal isolator, similar to the one shown below

What brand of chip is this? There are too many pins to fit.

QWE4562009

qwqwqw2088 posted on 2024-7-11 08:57 It would be much better to use digital isolator chips for isolation instead of optocouplers, such as ADuM1201 and ISO7221

Do I need two power supplies?

QWE4562009

damiaa posted on 2024-7-11 08:25 In both pictures, add a npn transistor to the side with the diode so that the driving ability is sufficient and it is not reversed. In addition, the communication rate must be high, and a high-speed optocoupler is required. If...

Moderator, please post a picture

QWE4562009

dukedz posted on 2024-7-11 09:24 When you use this chart, the data logic is reversed

How to change

QWE4562009

beyond_笑谈Published on 2024-7-10 21:04 Under high-frequency signals, PC817 must be replaced with a high-speed optocoupler, or a digital signal isolator, similar to the one shown below

Chuanshi Micro-Nano Chip Micro-Rongpai

beyond_笑谈

QWE4562009 posted on 2024-7-12 14:36 What brand of chip is it? There are too many pins to fit

Just check the specifications for the models in the picture. If they are not necessarily domestic components, many high-speed signal isolation chips

damiaa

QWE4562009 posted on 2024-7-12 14:38 Moderator, please post a picture

The 300 ohm resistor can be reduced appropriately or something like that. The resistor can also be adjusted directly.

It looks better to replace the triode with a double triode:

QWE4562009

damiaa posted on 2024-7-12 15:48 The 300 ohm resistor can be appropriately reduced or something. The resistor can also be adjusted directly. ...

3 sets of pictures are confusing

beyond_笑谈

If you don't use a high-speed signal isolation chip and want to use an optocoupler, it is recommended to choose a high-speed optocoupler, after all, it is used for serial port signal isolation.

QWE4562009

beyond_笑谈Published on 2024-7-15 17:21 If you don't use a high-speed signal isolation chip and want to use an optocoupler, it is recommended to choose a high-speed optocoupler, after all, it is used for serial port signal isolation.

What kind of optocoupler is a high-speed optocoupler? Which parameter should be higher than what level to be considered high-speed?

xmj0615

It would be better to use high-speed optocouplers for communication isolation optocouplers

QWE4562009

xmj0615 posted on 2024-8-6 13:49 It would be better to use high-speed optocoupler for communication isolation optocoupler

Digital chips are better