Why is the number of receiving ends in 485 communication limited and how is it calculated?

EEW2018

Why is the number of receiving ends in 485 communication limited and how is it calculated? [Copy link]

 

485 communication 1. How is the number of receivers 32 64 128 calculated? 2. When the voltage of A is greater than the voltage of B, is it 1 or 0? 3. When the transmitter sends 1, does it only depend on the voltage difference between AB or does the voltage of line A have to be greater than a certain V and the voltage of line B have to be greater than a certain V?

qwqwqw2088

These questions should be read through the "RS485 Protocol Standard"

You will find it in

qwqwqw2088

For example, the number of nodes. The "number of nodes" of RS-485 is mainly determined by the "receiver input impedance". According to regulations, the input impedance of the standard RS-485 interface is ≥12kΩ, and the corresponding standard number of driver nodes is 32.

The input impedance is designed to be 1/2 load (≥24kΩ), 1/4 load (≥48kΩ) or even 1/8 load (≥96kΩ), and the corresponding number of nodes can be increased to 64, 128 and 256.

qwqwqw2088

As for the 2nd and 3rd questions, the protocol standard has detailed descriptions, which are logic state issues.
The positive level between AB +2~+6V is one logic state, and the negative level between -2V~-6V is another state. At the
receiving end, the level between AB greater than +200mV is a positive logic level, and less than -200mV is a negative logic level.

EEW2018

qwqwqw2088 posted on 2019-10-25 09:01 As for the 2 and 3 questions, there are detailed descriptions in the protocol standard. They belong to the logic state problem. The positive level between AB +2~+6V is a logic state, and the negative level is -2 ...

Thank you very much, may I ask if 200mV is AB=200mV or BA=200mV?

As for the RS485 protocol standard you mentioned, I checked online and found about 20 or so documents all about 485 applications. Could you provide me with a download address for the RS485 protocol standard document?

Thank you so much.

qwqwqw2088

EEW2018 posted on 2019-10-25 09:19 Thank you very much, moderator. May I ask if 200mV is AB=200mV or BA=200mV? And what you said...

Download Place

Far away, yet close at hand

qwqwqw2088

Help the landlord find it

https://download.eeworld.com.cn/eewsearcher?kw=RS485%E5%8D%8F%E8%AE%AE%E6%A0%87%E5%87%86

topwon

EEW2018 posted on 2019-10-25 09:19 Thank you very much, moderator. May I ask if 200mV is AB=200mV or BA=200mV? And what you said...

For 200mV, is it AB=200mV or BA=200mV?

------AB>200mV, positive logic

BA>200mV, negative logic

The absolute value of the difference between A and B is between 200mV, which is an unstable state that needs to be avoided by design.

EEW2018

qwqwqw2088 posted on 2019-10-25 09:30 Please help the OP find https://download.eeworld.com.cn/eewsearcher?kw=RS485%E5%8D%8F%E8%AE%AE%E6%A0%87%E5%87 ...

Thank you so much.

EEW2018

topwon posted on 2019-10-25 11:05 For 200mV, is AB=200mV or BA=200mV? ------AB>200mV, positive logic ...

Thank you very much, but what you said seems to be exactly the opposite of what is in the information. Please see the picture below.

qwqwqw2088

EEW2018 posted on 2019-10-25 13:47 Thank you very much, but what you said seems to be exactly the opposite of what is in the information, please see the picture below

Based on the information you read

topwon

EEW2018 posted on 2019-10-25 13:47 Thank you very much, but what you said seems to be exactly the opposite of what is in the information, please see the picture below

Yes, I was wrong.

topwon

EEW2018 posted on 2019-10-25 13:47 Thank you very much, but what you said seems to be exactly the opposite of what is in the information, please see the picture below

I have some doubts about the source of your information. It is best to read the formal technical documentation.

topwon

EEW2018 posted on 2019-10-25 13:47 Thank you very much, but what you said seems to be exactly the opposite of what is in the information, please see the picture below

EEW2018

topwon posted on 2019-10-25 18:16

Thank you very much. Could you provide the download address of your information?

topwon

EEW2018 posted on 2019-10-26 07:24 Thank you very much. Could you provide the download address of your information?

AN-960_cn RS-485_RS-422电路实施指南 ADI.pdf (380.8 KB, downloads: 6)

2019-10-28 09:11 上传

点击文件名下载附件

EEW2018

topwon posted on 2019-10-28 09:11

Thank you so much.

I would like to express my sincere gratitude to all the teachers at eeworld for their great help and support. Thank you very much.