Hi, I have a project to monitor the battery voltage for each cell in series (36 cells, each cell 1.4V), after that each voltage data will be sent using an RS485 module. For each RS485 modbus slave I use an Arduino Nano and it will be sent to the master (Arduino Mega). After the Arduino Mega gets voltage data from each modbus slave. Arduino Mega will send the data via Modbus TCP/IP which will be monitored using Modbus Poll. The problem I had, when I only used 1 cell battery, communication ran smoothly (trial using 36 slave). But when I use 36 cell batteries in series. Then I used more than 5 slave (if use less than 5 slave communication ran smoothly) slaves for communication, no data was sent and the LEDs from the modbus module (tx, rx) were on. Below I attach the schematic that I use
Okay. Those modules have the 120 ohm resistor across the A/B bus connection(see device labeled "121", on the right hand side of the image). RS485 spec calls for one at each end of the bus, but NOT at each node. So you need to remove these termination resistors for some of your connections, but NOT all. As it stands, your bus has probably a half dozen of these resistors in parallel, which is loading down the signal, drawing too much current from the drivers.
See section six, here:
The third screw terminal on your 485 side of each module is for ground. That's what the chicken-scratch character represents, AFAIK. Without a common reference, your RS485 bus will work, until it doesn't ...
See grounding section in document already linked.
Now I have connected the terminal labeled "chicken-scratch" character to each connected RS485 module. But there was no change. Maybe what we need to pay attention here is "that when I use a 1 cell battery, communication runs normally, but if I use a series of batteries, communication will have problems". I suspect this is a wiring problem or some kind of noise if my batteries are in series
All the RS485 grounds need to be connected together, including the Master RTU.
However, once you do that, then all the battery negatives will be connected together, preventing you from measuring individual cells.
If the step-up converters are isolated then all the cell voltages will be isolated.
Good point, Jim. @CarakaTri You can buy opto-isolated versions of the RS485 interface. However, I cannot tell you, without perusing data sheets, whether they will operate for your intended purpose, as the isolation is primarily intended for noise rejection, not common-mode voltage rejection.
Isolated transceivers are available. Whether you'll find them as an Amazon, or other, vendor's packaged product I cannot say; a quick search yields a few results worth following.
Thank you @jim-p for your points. I will look for an alternative way to safely isolate the power supply used for the slave. Maybe that's a reasonable solution
Topic moved to a more suitable location on the forum.