I am planning a system consisting of 1 master device which is connected to 19 slave devices using RS485 transceivers. Also, all devices are powered by 1 PSU. A simplified schematic is shown below.
As you can see in the schematic, the slaves each control a 4W LED using PWM. I added this detail because I want to show that due to the switched loads a not unsubstantial current flows through the GND line. Therefore, due to the impedance of the GND line, the GND potential should be slightly different for each slave.
The bus should only be operated with a data rate of 250KBit/s. With a rise time of 180ns this would lead to a bandwidth of about 2MHz. However, I would also be very interested in what would happen if a data rate of 10MBit/s were used. With a rise time of 8ns this would lead to a bandwidth of 40MHz.
Since I am currently studying EMC aspects, I would be very interested to know the answer to the following question.
1.) How exactly do the current loops look when the master is communicating with the slaves?
While researching the exact operation of RS485, I came to the conclusion that the differential signal pair (A, B) of the transceiver are actually two single ended signals (Source 1, Source 2). This would mean that the current of the two signals would flow from the transmitter to the receivers and then back via ground. Thus, the outgoing and return paths of the current do not only consist of the signal lines A and B. So looking only on signal line A I came to the following current loop.
The red path is the outgoing current path and the green path is the current return path.
Since this current loop covers a larger area and permanently conducts higher frequency signals, I am concerned that this could cause some EMC issues. Also, at higher frequencies, the current return path may seek other unwanted paths back to the receiver, as the GND line is likely to have a significant inductive component. So, I would be interested in the answers to the following questions.
2.) How can I design a better current return path even for high frequencies?
3.) Are there other communication standards that might be better for this purpose?
Why don't I use isolated RS485 transceivers?
Isolated RS485 transceivers are not an option for economic reasons.
Why don't I use an additional GND line together with the A and B line?
On the one hand this would form ground loops and on the other the return current of the LEDs would then be divided between the two GND lines. This could cause a not insignificant current to flow through the master back to the PSU. This could not only cause disturbances in the electronics of the master, but could also exceed the maximum allowable current that the master board can conduct. Also, the additional ground wire would have to have a similar cross section as the other one. This is expensive and takes up a lot of space.
How is the data bus cable configured?
Since the slaves are very closely spaced and therefore the data bus cable has a stub every 10cm, a twisted pair cable without shielding is used.