Hi
I'd like to check if anyone more experienced can confirm my understanding of the VDC isolation the chip MAX3157 at the RS485 shield.
From the datasheet i can read it provides isolation from plus/minus 50 volts.
My understanding is that the signal pin can be carring that DC voltage in reference to the ground pin. That is, if my Grundfos pump runs on 85volts (two 40v solar panels in series plus safety margin), I won't be able to use this shield without adding further isolation, right?
Thanks
If the pump has RS-485 built-in it should be "safe".
If it's something you're building then you may have to "take precautions". Most optical isolators can isolate something like 1000V or more.
The Grundfos SQF 1.2-3 pump does implement RS485 out of the box. It can actually be installed deep underground (inside the well) and then add the 600€ complement to run the RS485 communications to read operational data (temperature, cause of last stop, if it was running dry, etc).
What i would like to implement is let my arduino mkr gsm 1400 know if the pump is running and the reason when it is not. Purchasing the oficial control device wouldn't let my arduino know that stuff and coordinate the irrigation times accordingly.
Is the isolation needed for safety?
Isolation for safety is typically done using opto or magnetic isolators with a separate isolated power supply. Slotted air gaps on the PCB are usually part of the design.
The datasheet for the MAX3157 looks like it's only designed to deal with ground shifts and ground loops.
Honestly, I don't follow. I thought the isolation was because the RS485 signal was running on top of the 85 VDC that actually power the pump. Communications share the same wires as power.
Normally RS485 is carried on a pair of wires separate from the power wires and the RS485 voltage at the pump should be 5v relative to ground.
Given the outdoor application and long wires an opto isolator is a good idea. It won't protect against lightning strikes but should protect against the 85v powering the pump under a fault condition.
There are just three wires used for both powering the pump and communications. Not my choice. That is the way the commercial pump is designed. It even supports to be powered with VAC and VDC (not at the same time, of course).
Does that help interpret the scenario?
Anyway, what is the purpose of the isolation at the shield then? Isn't it to decouple a DC voltage from the RS485 signal? Not sure what magic is performed when the pump is powered by VAC, but that is not my case.
I'm going to guess, but you need to confirm this:
2 wires are for power, with the data between the 3rd wire and one of the other two, is this correct?
The manuals that I found when I googled "Grundfos SQF 1.2-3" seemed to indicate that the three wires were Live, Neutral and Protective Earth.
(Or Positive, Negative and protective Earth when powered by DC voltage).
There was no mention of serial communications.
Could there be another variant with additional wires for RS485 communications?
I won't give a link to the manuals I found as they might be completely wrong. (but it was on the Grundfos website).
That is exactly what I was given. The same datasheet confirms the pump can do GENIBUS over RS485. And the installator confirmed there is no other pump to purchase or more wires to deploy when the controller is also installed. Actually they gave me the opportunity to add the said controller to my installation after lt was already finished, telling me it would require only to add the device to the box you can see at the picture.
I didn't realize it could be using ground and negative pin to transmit the RS485 signal. Obviously the DC voltage is transmitted between positive and negative. That would leave ground floating, i guess. I should check again the voltages between the three pins.
Thanks for the idea, @PerryBebbington.
Anyway, the question was what the isolation actually means for the RS485 shield. Is it then that the two pins used can carry up to 50 vdc then the serial signal on top?
I did some searching for GENIBUS, what I found is that it is indeed a 2 wire RS485 connection. While I could not find out how it connects to a pump I found enough to be confident that it doesn't connect RS485 directly to the power wires. My guess is that there is some additional hardware that somehow takes the RS485 data and imposes it on the power wires. How it does that will be proprietary to Grundfos. You need to find out first it I am right and then if I am find out what that hardware is.
Note I have edited the above to be a little bit clearer.
It can't. Look at the order of the replies. After I suggested that it became clear that the 3rd wire is earth. I didn't know that when I made that suggestion.
MY research shows the use a data over power lines communication that requires the use of their transceivers. Search for the term "genibus".
You mean that other than this https://github.com/christoph2/GENIBus library and the RS485 shield, I must use a propiatory hardware device?
I thought it was capable of talking RS485 according to the datasheet, and that covered the hardware side of this integration. Leaving the software part to the GENIbus library.
May be the library reads from the adapter you are referring to?
Based on what you have shown us you don't have any hardware with an RS485 interface. I see only a pump with power wires to it. Where do you think you connect the RS485 bus? It can't be the power wires, so it must be something else. I don't see the something else in anything you have shown us. Have I missed something?
A schematic showing all your hardware and how it is wired would make helping you a lot easier. Also show how you think the RS485 bus would be connected.
I cannot find documents in English, but this is the conection for the standard control device named CIU 273 SQFlex GRM. As you can see there is no other wire there than those for power.
I will try to draw the schematics, but I am still figuring out myself. The box is quite messed up and i am not sure if I am drawing it correctly.
From my understanding, there are the DC wires coming from the solar panels providing 82VDC and 11A maximum. Those fork in two to feed a 12VDC converter and the power terminals of a power switch ("contactor" in Spanish). The power switch is controlled by the output from the 12VDC converter that goes through a water level switch. After that power switch there is a mechanical switch that allows me to cut open all three wires, adding gnd there. Then the three wires into the pump. There is where I would connect my RS485 shield pins.
Will draw it anyway. But it will take me a while. Please, be patient.
From the same document, in Spanish
La comunicación entre la CIU 273 y la bomba se realiza mediante el cable de suministro eléctrico de la bomba. Esto se llama señalización mediante borne de la red (o comunicación a través de cable eléctrico) y este principio significa que no son necesarios cables adicionales entre la CIU 273 y la bomba.
I decided against translating it myself or use Google translator trying to keep the source as available as possible.
It literally says the control runs on the same wires as power.
https://api.grundfos.com/literature/Grundfosliterature-4347792.pdf
Which confirms what I, @JohnLincoln and @Paul_KD7HB have said, the connection to the motor is not RS485, it is a something proprietary to Grundfos. In order to link it to RS485 you need their interface that provides an RS485 connection. RS485 does not connect to power wires, only to other RS485 devices.
