The most underestimated factor in serial device networking transformation is distance. The cables on the drawing are only tens of meters long. After actually passing through the power distribution room, on the bridge, and across the building, interference and voltage drop will cause the originally stable 485 bus to frequently lose packets. This article gives judgment and processing ideas.

First determine whether the bottleneck is the transmission distance

The empirical bus length of RS485 at 9600bps is about 1200 meters, but this is an ideal value: when the wire diameter is too thin, there are too many branches, and the surrounding frequency converters are densely packed, the effective distance will be significantly reduced. It is recommended to use a multimeter to measure the terminal matching resistance and common mode voltage first to confirm that the bottleneck is distance and interference, and then talk about link upgrades to avoid replacing equipment in vain. If the measured voltage difference at the edge node is obviously too small, it can basically be determined that the problem is distance.

Across floors and buildings: use optical fiber to extend 485

It is recommended when the distance exceeds 100 meters or the on-site interference is strong2-way bidirectional RS485 data optical terminalThe two buses are transparently transferred to the optical fiber bit by bit: one is placed at each end, and the protocol, baud rate, and host computer software are all left untouched; the optical port naturally forms electrical isolation, which is particularly effective in suppressing lightning strike induction and ground loop interference.

Implementation steps: Three details determine success or failure

• The optical transceiver draws power nearby, and the pigtail is laid away from the power cable;
• The bus-side terminating resistors are still only placed at the physical ends;
• Before the official switchover, perform short-range verification for one day, and then switch to the long link.

There are mature practices for long-distance monitoring scenarios such as high-speed tunnels and pipe corridors. You can refer toOptical fiber project for vehicle monitoring system in highway tunnel in a province;For overall planning ideas, seeIndustrial automation and machine vision network solutions, please contact EB-LINK engineers for extended distance calculation.