Six RS-485 analyzers in different panels: one serial line or one gateway per panel?

[PlantInsight]

We're adding six energy analyzers with RS-485 (Modbus RTU) to distribution panels in the same building. The panels are between 20 and 120 m apart. There is an Ethernet switch near two of the panels, but not near the others.
Our electrician suggests one RS-485 cable daisy-chained through all six panels to a single serial-to-Ethernet gateway. Another option is a gateway in each panel. Which is the better approach for reliable energy data, and what should we watch out for during installation?

[Technical Support]

Both approaches work. One shared serial line is cheaper and simpler to maintain if it's installed correctly. A gateway per panel (or per group of panels) costs more but isolates problems and scales better. For six analyzers in one building, a single line or two short lines is usually fine.
If you go with one RS-485 line
1. Daisy-chain, no star. Run the cable from panel to panel. Avoid long branches (stubs) to individual analyzers.
2. Use proper cable. Twisted pair, shielded cable intended for RS-485. Keep it away from power cables where possible and cross them at right angles.
3. Termination at both ends only. Termination resistors belong at the first and last device on the line, not on every analyzer. Many analyzers and gateways have a built-in termination switch.
4. Shield and ground. Ground the shield at one point only, to avoid ground loops between panels.
5. Unique Slave IDs. Give every analyzer a different Modbus address before connecting it to the line, and write them on a label in the panel.
6. Same serial settings everywhere. Baud rate, parity and stop bits must be identical on all six analyzers and the gateway.
If you use more than one gateway
- Group panels that are close to each other or close to the existing switch.
- Each gateway gets its own IP address. Put them on the network segment intended for field devices, not the office network.
Gateway mode
Check whether the gateway works as a Modbus TCP to RTU gateway or in transparent (raw TCP) mode. The software side must be set up to match. Mixing the two is a common reason for timeouts even when the wiring is correct.
Commission step by step
Connect and verify one analyzer at a time. Compare its values with the analyzer display before adding the next one. Once all six are on the line, watch for a day for intermittent timeouts. On a shared line requests are answered one after another, so a polling interval that's too short for six devices can cause errors that look like wiring faults.
Background:
- https://greensoft.ai/blog/modbus-rtu...communication/
- https://greensoft.ai/blog/modbus-tcp...communication/