What factors restrict the number of receiving terminals in the 485 communication mode of 1 send and 32 receive? Is it the distributed capacitance?

EEW2018

What factors restrict the number of receiving terminals in the 485 communication mode of 1 send and 32 receive? Is it the distributed capacitance? [Copy link]

I would like to ask what factors limit the number of receiving ends in the 485 communication mode of 1 send and 32 receive? Is it the influence of distributed capacitance on the waveform?

qwqwqw2088

485 communication 1 send 32 receive belongs to differential mode,

Multiple nodes require terminal matching

chunyang

It is determined by the input impedance of the receiver. You can understand this by looking at the device manual of the 485 interface chip with more than 32 nodes.

topwon

The most critical parameter is the UL value (unit load) of the 485 chip, which is the input impedance (12Kohm) mentioned above. The 485 bus standard is to drive 32 ULs. If the selected 485 chip is 1/4UL, then the upper limit of the number of devices that can be driven by one bus is 32x4=128.

Secondly, it is related to the bias resistance value designed in your actual circuit, the bias pull-up voltage, and whether the terminal matching resistance is connected (so it is also related to the baud rate and distance. If the baud rate is low and the distance is short, there is no need to add terminal matching resistance).

EEW2018

topwon posted on 2019-9-16 16:44 The most critical parameter is the UL value (unit load) of the 485 chip, which is the input impedance (12Kohm) mentioned above. The 485 bus standard is to drive 32...

Thank you very much, but I still want to ask, if the receiver is a quarter of the UL, then it is 12K ohms/4=3K ohms, then the current on the 485 line will increase by 4 times, and the current of each of the 128 terminals will increase by 4 times, so the total current will increase by 128*4 times, which is incredible. Am I understanding it wrong?

topwon

EEW2018 posted on 2019-9-17 07:35 Thank you very much, but I still want to ask, if the receiver is a quarter of UL, then it is 12K ohms/4=3K ohms, then the current on the 485 line will definitely increase by 4 times, so 12 ...

The input impedance corresponding to one quarter of UL is 12Kx4=48K ohms. . . . .

EEW2018

topwon posted on 2019-9-17 11:14 The input impedance corresponding to one-quarter UL is 12Kx4=48K ohms. . . . . .

Thank you very much. How to add matching impedance at the end of the line, how to calculate the value of matching impedance, and are there any construction pictures of adding matching impedance? Thank you.

topwon

EEW2018 posted on 2019-9-17 11:39 Thank you very much. How to add matching impedance at the end of the line, how to calculate the value of matching impedance, and are there any construction pictures of adding matching impedance, thank you...

If the comparison between the cable length and the baud rate results in the need to consider it as a transmission line,

The simplest termination method is to connect matching resistors in parallel at both ends of the cable (if it is a unidirectional 485 network, then just on the far end). The value of the matching resistor should be consistent with the characteristic impedance of the cable used (485 cable is generally designed according to 120 ohm characteristic impedance).

It is important to remember that even if you need to add matching resistors, you can only add them at both ends of the entire cable and not at other nodes.

topwon

RS-422-RS-485-eBook.pdf (5.02 MB, downloads: 7)

If you are interested, you can study this application document carefully. When is termination required, how to terminate, how to bias, etc.

topwon

RS-485 basics - The RS-485 Driver.docx (68.61 KB, downloads: 0)

RS-485 basics - how to calculate unit loads and the maximum number of nodes on y.docx (101.51 KB, downloads: 1)

RS-485 basics - the RS-485 receiver.docx (155.54 KB, downloads: 0)

RS-485 basics - two ways to fail-safe bias your network.docx (62.87 KB, downloads: 0)

RS-485 basics - When termination is necessary, and how to do it properly.docx (105.62 KB, downloads: 0)

These are some reference materials I collected from TI's official website. You can also study and discuss together.

What factors restrict the number of receiving terminals in the 485 communication mode of 1 send and 32 receive? Is it the distributed capacitance? [Copy link]

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