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My Raspberry sends data at baud rate 115200 to 300 remote panels (ATmega128A) through RS485 bus serial communication. Also it sends enable signal (5 V) and receive feedback from the 300 panels at the same baud rate.

I distributed every 15 remote panels on one group of bus (data, enable and feedback) the number of bus groups is 20 groups so the total remote panels is 300 (check system diagram below).

System is tested in the pilot using only 2 panels to reduce testing cost! Now, I'm going into production and want to make sure that the following will work fine:

  1. Using 74244 buffer is suitable for multiplying Raspberry data steam and enable signal 20 times for 20 groups of bus as in the diagram.

system diagram

  1. Using Zener 5V to (OR) the 20 feedback steams in one input pin to the Raspberry.

  2. Is it better to power the buffer from the Raspberry power supply (Vcc-ras) or from the MAX485 power supply (Vccmax) on the mother board?

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  • \$\begingroup\$ Can you put in a schematic drawn with the tool? \$\endgroup\$
    – Voltage Spike
    Nov 26, 2023 at 22:11
  • \$\begingroup\$ production […] data steam vaporware? \$\endgroup\$
    – greybeard
    Nov 27, 2023 at 8:42
  • \$\begingroup\$ What is the point of one and the same enable signal to all 300 panels? What is to be the distance to the furthest panel? Why not WLAN? \$\endgroup\$
    – greybeard
    Nov 27, 2023 at 8:59
  • \$\begingroup\$ What's the point of the enable? What if one panel is unpowered and it shorts the enable to 0V, no other panel in that group will work. Also why have full duplex return path if only one of the panels can drive data to feedback bus; would a single half-duplex bus work? You also don't say which 74244, really a 74244 or will it be at least a 74LS244, or 74HC244, 74AC244, or something else? Also if you are not aware, RS-485 is not a two-wire bus. It sure has two data wires, but all transceivers need a common ground reference to work. \$\endgroup\$
    – Justme
    Nov 27, 2023 at 9:18
  • \$\begingroup\$ please recommend an eas drawing tool \$\endgroup\$
    – Ezzat
    Nov 27, 2023 at 19:10

2 Answers 2

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I don't see any advantage to an individual driver for each "data MAX485".

Instead, let the ATMega drive two drivers for data, each driving 10 differential drivers,
and two more drivers for enable, each driving 10 more drivers for a total of 24 - 3 packages with octal drivers.
Select drivers according to driving capacity and ruggedness required.
There are multichannel RS-485(/422?) transceivers.

Driving capacity of '244 varies depending on logic family; not all would be up to driving a 50 Ω coax.

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  • \$\begingroup\$ thank you bro that is much helpful \$\endgroup\$
    – Ezzat
    Nov 27, 2023 at 18:40
  • \$\begingroup\$ would you please advise for OR the 20 feedback input to the Rpi \$\endgroup\$
    – Ezzat
    Nov 27, 2023 at 18:41
  • \$\begingroup\$ You can build big gates starting with smaller ones. For an OR, don't just look at OR gates: Start with NOR, even levels get NAND. Details depend on logic family. \$\endgroup\$
    – greybeard
    Nov 27, 2023 at 18:59
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1. Using 74244 buffer is suitable for multiplying Raspberry data steam and enable signal 20 times for 20 groups of bus as in the diagram.

Probably not. I'll be surprised if a RPi3 pin can drive that many TTL inputs. Can you actually get 74244 these days? Or do you mean something like 74LS244 or 74HCT244?

2. Using Zener 5V to (OR) the 20 feedback steams in one input pin to the Raspberry.

Again, probably not. Diode-ORing, especially with a 10k pullup is not going to be very fast. Have you tried it? And why are you using zeners instead of ordinary signal diodes, or better yet, Schottky diodes?

3. Is it better to power the buffer from the Raspberry power supply (Vcc-ras) or from the MAX485 power supply (Vccmax) on the mother board?

Neither. The RPi3 I/O pins are only rated for 3.3 V.

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  • \$\begingroup\$ Im not sure which 244 is available but i will check \$\endgroup\$
    – Ezzat
    Nov 27, 2023 at 18:42
  • \$\begingroup\$ 1- I'm not sure which 244 is available but i will check. 2- i tried zener for few number of of panels. and actually I copied this zener design from other schematic. 3- i mean the power supply of either Rpi or MAX IC not the output of them \$\endgroup\$
    – Ezzat
    Nov 27, 2023 at 19:08

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