4
\$\begingroup\$

I'm new to community and it's my first question, so please forgive me for mistakes I could do referring to adding pictures and descriptions.

I plan to use 100 devices and connect them via RS-485 in half duplex manner as it can be seen in below picture. Each node will need max 150 mA and I plan to use 12V 20A power supply connected to 220V AC main. Termination resistors will be added to ends.

enter image description here

I searched for problems I might end up with RS-485 communication and saw that ground loops might be problematic. In that sense, implementing specific ground for communication and isolation is offered as it can be seen below picture;

enter image description here

However all of examples I could find related to ground loop problems in RS-485 mentions about ground differences according to different power supplies connected to building mains which are far away and might have different potentials although power supply reference is earth. Connecting grounds might create ground loops and interfere with communication signals. Large current might created in ground loop and might damage the circuit. If grounds are not connected, than ground potentials might be a lot different and transceiver could not handle ground potential gap.

In my case I will use single supply and connect all devices in daisy chain. And in worst case total of 15A will be used and I am afraid this might also create ground differences between devices. Would this will create ground loop like in the case with different power supply? I have no experience about transmission line but in my PCB designs I try to seperate digital grounds and analog grounds and also seperate high current grounds and connect them at one point to avoid ground loops. By the way, total distance (device 1 to 100) will be no more than 200 meters and baud rate is low (9600). I know isolation is the best way but from cost perspective this setup is my preferred one if there will be no problems.

Also I would like to ask about fail-safe pull up and pull down resistors connected to A and B lines. Having them only at the master device is enough or should I have them at every device?

Thanks for replies,

Mert

\$\endgroup\$
6
  • 2
    \$\begingroup\$ A very warm welcome to the site and thank you for putting a lot of effort into such a well-written question, upvoted. On the fail-safe, the resistors would be to put the system into a safe state. So where you need them depends on where you think connections can get broken. That's something for you to think through. By the way, please edit new information into your question rather than add it in comments, and not under an 'Edit' banner - rewrite the text as needed instead. Again, welcome. \$\endgroup\$
    – TonyM
    May 27 at 9:13
  • \$\begingroup\$ Thank you for the warm welcome message and your reply. So I think if there will be no hot plug or no disconnection is assumed, having at only one node would be enough for having no undefined state when all nodes are inactive. \$\endgroup\$
    – mertkon
    May 27 at 9:28
  • \$\begingroup\$ one way around ground loops is to magnetically couple the signal in and out of each device. Cheap transformers for that are indeed available, because every ethernet card has one. That, however, might point to the fact that at the size of your system, ethernet might become easier to handle (and financially comparable) than RS485. \$\endgroup\$ May 27 at 9:33
  • \$\begingroup\$ General question: you seem to indend to use isolated supplies for each local supply (I guess that is what you mean with "supply isolator"); is that really necessary? (also: if you do that, you can use a higher voltage and lower current on your supply lines, thereby reducing the cable losses.) That sounds pretty expensive, and unless you're expecting significant ground shift, the whole idea behind RS-422/-485 was that the differential signaling was robust against small ground shifts. \$\endgroup\$ May 27 at 9:34
  • \$\begingroup\$ The proposed design in the second picture is suggestion from Texas Instruments paper. I also don't think it is necessary and also in that example there are different power supplies to each node connected to mains. In my case there exist one power supply with 12V and I could not be sure whether I end up with communication problem. Also I could increase the voltage to 24V to lower current to half as you pointed out but I have to lower 24v to 5v inside each node, that will cause heat but I think it won't be problem with switch mode regulators like lm2596 \$\endgroup\$
    – mertkon
    May 27 at 9:47
1
\$\begingroup\$

To overcome those problems, you should start to think about using isolated transceiver. Additionally you would need few RS485 repeaters, because a tiny 485 transceiver can't drive 100 devices at once.

By the way, you don't connect the PE directly on transceiver, it's just for the ESD protection potential. If integrated into IC then yes, you connect it to GND of the secondary,... there are various possibilities best if implemented as declared from manufacturer.

Example 1 You can see GND3 is floating, tied to earth potential though a capacitive coupling 4.7n and by means of high resistance 1M.

enter image description here

Example 2 Same manufacturer, now the earth potential is connected directly to IC. Note: In both circuits only A and B are connected to the other node.

enter image description here

\$\endgroup\$
5
  • \$\begingroup\$ In MAX3078 datasheet, it is said that transceiver is 1/8 unit loads so therotically 32*8 = 256 nodes are allowed but I might be wrong. About isolation yes you are right isolated transceivers or implementing isolation circuit is the best way but this project will be used as Pick By Light system in warehouse and there will be around 1000 nodes group of 10 consist of 100 nodes. I might also reduce groups to 50. Minimizing the cost is ultimate goal and if someone who did likewise setup says there will be no problems regarding to communication I want to use cheap MAX3078 solution. \$\endgroup\$
    – mertkon
    May 27 at 9:39
  • \$\begingroup\$ @mertkon It's up to you to decide. If 1000 nodes will break, then it will be very expensive solution. \$\endgroup\$ May 27 at 9:51
  • \$\begingroup\$ Yes you are right indeed. If there will be any chance of a problem and system not work than it will be a lot more expensive. As the saying goes "I'm not rich enough to buy cheap things" \$\endgroup\$
    – mertkon
    May 27 at 9:57
  • \$\begingroup\$ @mertkon I took a look in datasheet for MAX3078 and you better use a slew rate limited transceiver if you intend to use lower speeds than several Mhz, it would perform better. Unfortunately all app. notes from MAXIM using those ICs are showing isolated type. \$\endgroup\$ May 27 at 20:22
  • 2
    \$\begingroup\$ @mertkon You could even use an industrial standard 24VDC for supply , then an isolated 24/5V converter then you can interface MCU and 485 IC directly. Then the whole board will have a floating potential. \$\endgroup\$ May 27 at 20:46
1
\$\begingroup\$

Galvanic isolation isn't necessary when every node in the network runs off the same power supply.

Many people run at least 5 wires to each node, including 2 separate independent "GND wires" to each node: the "Power GND" wire (typically paired with the "+12 V power" or "+24 V power" wire), and a separate "signal GND". Many communication buses are called a 2-wire bus, but actually recommend 3 wires (a differential pair + a ground reference), including RS-485, CAN bus, Profibus, etc. 1 2 3 4 5 6 7 8

The signal ground is typically connected with a 100 Ohm resistor to the power ground at each node. ( RS485 Common Ground: Common Voltage or Common Wire or Both? ) 9 10

The recommended fail-safe and termination resistor arrangement has 4 resistors:

  • 2 termination resistors, one at each end of the long linear RS-485 bus, directly across the A and B wires. Ideally the characteristic impedance of whatever cable you are using, which is typically around 120 Ohms or 150 Ohms. And,
  • 2 fail-safe (biasing) resistors, one pulling the "-" wire down to "signal GND", the other pulling the "+" wire up to data power. (Data power is most often +5 V but I've seen +3.3 V in some systems). It doesn't matter where the bias resistors are located. Many systems put them only at the leader node, because none of the follower nodes need them. Many other systems put them only at one end of the linear RS-485 bus (it doesn't matter which end), next to the termination resistor for that end. 11 12 13

A few systems (for symmetry, so both ends of the network are the same) use 6 resistors ("dual fail-safe biasing"), 3 at each end, which arguably works slightly better than 4-resistor termination:

  • 2 termination resistors (same as above). And,
  • 4 fail-safe (biasing) resistors, two at each end of the linear RS485 bus. Two of them -- one next to each termination resistor -- pulling the "-" wire down to "signal GND". The other two of them -- one next to each termination resistor -- pulling the "+" wire up to data power. Each of these 4 resistors have double the resistance of each of the two biasing resistors of the 2-bias-resistor systems. These 4 resistors are typically around 1 KOhm each. 14 15

Some transceivers may not need external biasing resistors, either because their receiver threshold excludes 0V, or they already include internal failsafe biasing resistors. 16 17

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.