Retrofit without modification
Read the RS-485 bus without touching the PLC program.
On a lot of machines the PLC program is off limits: the OEM will not release the source, the warranty is void if it is modified, or the line cannot be stopped long enough to change it. Listen-only mode exists for those cases. The panel connects to the existing bus in parallel, reads the traffic, and never transmits.
The situation it solves
OEM machine, closed program
The machine builder will not hand over the PLC source, and will not add a display for you.
Warranty on the line
Any modification to the PLC program or the existing bus voids the machine warranty.
No engineering access
Nobody on site has the software, the password or the authority to change the program.
Mixed vendor line
Several machines from different builders, each with its own PLC and its own rules.
How it works
RS-485 is a shared two-wire bus. Every frame the master sends and every reply a slave returns is visible to anything listening on those two wires.
- Connect in parallel. The panel's A/B lines join the existing RS-485 pair. No device is removed, no address is changed, no parameter is written.
- Match the frame format. Baud rate, data bits, parity and stop bits are configured to match the bus. The panel then recognises request and response frames.
- Map the registers you want. You tell the panel which slave address and which register offsets hold the values. Those values are rendered on the display.
- Never transmit. The panel does not poll, does not answer, and does not take a Modbus address on the segment.
Because nothing is written, the PLC program has no way of knowing the display exists. The worst case is a panel that shows nothing; it cannot corrupt a running process.
What it cannot do
This matters more than the feature list. Read it before ordering.
| Limitation | What it means in practice |
|---|---|
| The data must already be on the bus | If the PLC never asks for a value, or no slave ever sends it, the display cannot show it. Listen-only observes; it cannot request. |
| The protocol must be Modbus RTU | A proprietary or vendor-specific protocol on the same wires cannot be interpreted. We can check a capture before you commit. |
| Update rate follows the bus | The panel refreshes as often as the value appears on the bus. If the master polls a register once a minute, the display updates once a minute. |
| Electrical care is required | Adding a node loads the bus. On long segments or heavily loaded networks this needs checking rather than assuming. |
| It is read-only, by design | There is no path to write setpoints or commands back to the PLC. If you need to send values, use master or slave mode instead. |
Before you order
Listen-only installations are decided by evidence, not by the datasheet. Three things settle it:
- A short RS-485 capture from the live bus — we confirm it is Modbus RTU and identify the slave addresses.
- The register addresses holding the values you want to display.
- The baud rate and frame format, and the cable length between tap point and panel.
If the capture shows the values never travel on the bus, listen-only is the wrong mode and we will say so.
Send us your bus details