i'm doing some researches about how to close/open RS485 data line in my RS485 multi device bus.

I'm following this nice research-document: Link Here.

Basically i'm thinking how to solve this problem in a real-world environment, as you can see in page 43, the strategy is very simple: every slave, using a microcontroller and a relay, will have A-B data lines normally closed, and when the master want to set/reset/reconfigure the slaves address it will manage the relay coil and sends to the respective slave the "configuration" messages.

Basic concept of connections

I wish to receive some advices in how i can manage the RS485 data lines using a low cost and effective solution without using a relay: my goal is to develop the same system using smd components.


I have 10 contact sensors that are placed around my home, they monitor the door open/close status and using RS485 trancievers the slaves (that are equipped with an ATMEGA328P) will expose those information to the master of the bus. Now, i want to be free to position those devices in my home without worrying to set a slave address hardcoded in my firmware or using hardware dip-switches.

They are connected using a CAT5e cable and the total length is around 20 meters, the cable will transport: A-B-VCC-GND.

Every device is wired in daisy chain mode and the preceding device will control the power up of the following sensor, using a mosfet on GND controlled by the MCU (as reported in the image below):


I don't know if that can really work: the RS485 data lines are connected anyway, also if the circuit is not powered on...

Basically, this "sequential power up" is intended to solve the issue to know "where (for example) is located the device with id 3?"

Any suggestions?

  • 3
    \$\begingroup\$ Sounds like an XY problem. Explain why you need to disconnect a 485 interface from the AB line using a relay? \$\endgroup\$
    – Andy aka
    Jun 19, 2020 at 14:39
  • 2
    \$\begingroup\$ I don' think this will work because un-powered devices may load the bus on A and B and upset the standard 120 ohm cable termination impedance at the end of the cable. If you used relays to partition the bus, the end terminator can no longer be on the bus current accessed and data reflections/bad data will likely happen. You would also need to have switchable line terminators on each slave. How would "location" information be known? \$\endgroup\$
    – Andy aka
    Jun 19, 2020 at 15:39
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    \$\begingroup\$ No, you do not need to dynamically connect or disconnect the RS485 lines. Either a) add an extra wire which is an input from the previous system and and output to the next, so that the devices can "count off". Or b) Connect as RS422 instead (distinct TX and RX) and in a loop rather than a bus and repeat messages onwards. Look at how LED strips "strip off" a unit of data before passing it on. \$\endgroup\$ Jun 19, 2020 at 15:40
  • 1
    \$\begingroup\$ @ChrisStratton Is that worth the extra wire running around your house though? I've tried the message forwarding scheme in one of my own machines and though it did work, I would not recommend it since it had its issues with passing on long messages and would hiccup sometimes interrupting the message passing midstream. \$\endgroup\$
    – DKNguyen
    Jun 19, 2020 at 15:42
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    \$\begingroup\$ @DKNguyen the OP's proposal already has the extra wire, it's the relay that's unecesssary. That said I'd personally just put labels on them, walk around with a notepad and assign those addresses to locations at the server. Or for a product QR codes and an app to scan them and chose "living room" or whatever. Your hiccuping sounds like UART buffering code issues, and also perhaps not putting some inter-message idle time in at the origin in case the embedded module's baud rate is a little slower than the master's. \$\endgroup\$ Jun 19, 2020 at 15:43

1 Answer 1


You will need a switch of some sort. Maybe a relay, but an analog mux is probably better if you can find a suitable one (low enough resistance and rugged enough for RS-485 (aka high working voltage and ESD protected).

If you can't find higher voltage switches with low enough resistance, you may need to add a repeater in each slave so that the series resistance of the switches doesn't accumulate.

The sequence (which I concocted just now) that happens is this:

  1. All slaves boot up with the next slave connected to the chain.
  2. Master sends a "count-in start" command that is received by all slaves.
  3. In response to the "count-in start" command, all slaves go into a mode expecting count-in communications from the master and disconnect the next slave from the chain leaving only the first slave connected to the master.
  4. Master communicates with the first (and only connected) slave and counts it in.
  5. Upon completion of count-in, the first slave connects the next slave to the chain and waits for a "count-in finished" command. Otherwise it ignores all messages.
  6. The master starts sending count-in messages which are only responded to by the most recently connected slave (since all the others are either disconnected or ignoring everything except for the "count-in finished" command) and counts itself in.
  7. Upon completion of count-in, the slave connects the next slave up and waits for a "count-in finished" command. Otherwise it ignores all messages.
  8. Repeat steps 6 through 7 until all slaves are counted in. You can detect when there are no slaves left with a time-out.
  9. Master sends "count-in finished" command which causes all slaves.


simulate this circuit – Schematic created using CircuitLab

The way the temrination resistors are portrayed here isn't quite what it needs to be for RS-485. But hopefully it gives you an idea of how to use the DPDT switch to disconnect the next slave while swapping it in for a termination.

I do not recommend using the RS-422 method (mentioned by others) where there is only a dedicated bus running between two adjacent slaves and there is no global bus running to all slaves. I did this on an old project of mine so that the slaves could count themselves in and though it did work, there were problems.

First, it was very slow to propagate messages down the chain in a game of telephone.

Second, sometimes hiccups would happen and messages would stop propagating down the chain mid-way, especially when the system was busier doing other things). It was a DMA, interrupt driven system and I was never able to pinpoint what exactly caused this to be able to fix it. I ended up adding message verification and sending the command in multiple times but never did find the root cause.

Third, it could not repeat long messages down the chain (i.e. data logs) because they the message would get interrupted. I don't know why this happened to this day since the DMA buffer should have been able to handle it. Then again, I can't remember if it sent complete data logs for the slave nearest to the master. It might not have been a fault of the forwarding down the chain.

  • \$\begingroup\$ Many, many thanks! now i'm searching about the analog mux that can handle the slave RS485 "switch". you think that a CD4066 can be suitable for that? you have some alternative to suggest? \$\endgroup\$
    – VirtApp
    Jun 19, 2020 at 16:23
  • \$\begingroup\$ Maybe i've found the one: TS3A24159 - ti.com/lit/ds/symlink/ts3a24159.pdf - What do you think? \$\endgroup\$
    – VirtApp
    Jun 19, 2020 at 16:30
  • \$\begingroup\$ I would try and find a higher voltage analog switch to deal with the unpleasantness that RS-485 is expected to experience. Like 15V. But 15V analog muxes also have higher series resistance which could become a problem when you have a bunch of them in series. You will have to judge for yourself since I have not actually built a system like. I'm not sure how much series resistance RS-485 can handle. \$\endgroup\$
    – DKNguyen
    Jun 19, 2020 at 16:32
  • \$\begingroup\$ There is a chance you might have to just add a booster/repeater in each slave to prevent cumulative series resistance from all the switches. Leave room on your PCB for this and add jumpers to bypass it in case you don't need it. \$\endgroup\$
    – DKNguyen
    Jun 19, 2020 at 16:37
  • 1
    \$\begingroup\$ Using such a switch will defeat the ruggedness of RS-485. \$\endgroup\$
    – DKNguyen
    Jun 19, 2020 at 16:43

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