Issues with RS485 Repeaters and Profibus Signal Voltage

There have been persistent issues with Profibus communication at a certain brewery. Previously, SIAS and on-site engineers had re-planned the entire Profibus network topology, but there were still instances of DP slave devices dropping out, occurring multiple times in a short period, severely affecting the factory’s normal production. Therefore, the user hoped to conduct a thorough diagnosis of the on-site issues.

Since previous engineers had already made multiple attempts to resolve the issue, it became quite tricky: on one hand, it was technically challenging because all possible measures had already been taken on-site, yet none seemed to solve the problem; on the other hand, if not handled properly, it could lead to doubts about our product quality from the customer, hence the pressure in handling similar issues is considerable.

However, “regardless of whether you handle it or not, the problem is still there,” so during the customer’s downtime for maintenance, we still went on-site to diagnose the issues again.

After actual inspection on-site, it was found that the problems were mainly concentrated at the last few stations in each network segment, some were ET200M, and some were third-party valve islands. Due to the high temperature and humidity in the on-site environment, I first suspected cable aging or loose connections. However, the user reported that most cables had been replaced recently and were re-wired, so this didn’t seem to be the issue.

Next, I used an oscilloscope to check the Profibus waveform on-site. I found that the signal waveforms from all problematic slave devices were relatively good, except for one point: the voltage was too low.

▲ Figure 1: The main station signal voltage measured at the terminal of the network segment is too low

From the waveform inspection, it can be seen that at the problematic stations, the maximum voltage of the main station signal was only 3.28V, whereas the Profibus signal waveform generally should not be lower than 4V; otherwise, proper communication cannot be guaranteed. Therefore, the on-site issue is likely due to the main station signal voltage being too low, leading to the slaves being unable to recognize the main station signal. Hence, we first need to increase the amplitude of the main station signal voltage.

Accordingly, I suggested adding an RS485 repeater to the bus, but in reality, repeaters had already been added on-site, and the installation position was only a few dozen meters from the last station, well within the specified range. “So why is the voltage still so low?”

Thus, I measured the main station signal before the repeater and found that the main station signal was also very low before the repeater. Therefore, it is highly likely that the main station signal was already very low before reaching the repeater, causing the repeater itself to fail to correctly recognize the main station signal, leading to frequent dropouts of the connected slave devices. Hence, I suggested adjusting the position of the repeater and retesting.

Unexpectedly, this suggestion exhausted both me and the on-site engineer. To find a suitable installation position, I worked with the on-site engineer to test various slave device locations. Since most of the devices in the network segment were third-party valve islands, the Profibus connectors were internal to the valve islands, and the design of the valve islands used M12 connectors, making it very difficult to disassemble and connect due to limited space on-site.

▲ Figure 2: Wiring of valve island equipment

After numerous disassemblies, connections, and tests, we finally discovered that due to the high power consumption of the valve island equipment, the voltage would drop significantly after passing through 5 valve islands. Therefore, we allowed a maximum of 5 valve islands per network segment. After a series of modifications, we finally increased the voltage at the last station to above 4V.

▲ Figure 3: The main station signal voltage measured at the terminal of the network segment is now normal

After the system modifications, it finally resumed normal operation, and after a period of running, no further issues arose. Therefore, it can be seen that although repeaters were installed on the bus, whether they are used correctly still has a significant impact on bus communication. Thus, from the handling of this on-site issue, it can be seen that field engineers should understand and master the correct usage of RS485 repeaters. Additionally, the correct topology on-site is also very important; without the previous engineers’ modifications to the topology, it might have taken much longer to identify the problem.