Compatibility
Modbus TCP and Modbus RTU driver for SCADA
Nexus reads any Modbus device with two drivers: modbus-tcp over Ethernet and modbus-rtu over a serial line. Both are included in every licence.
What it connects to
Anything with a Modbus interface: drives, meters and controllers. Schneider Modicon controllers speak Modbus, so the checker points them here. The same drivers are the fallback for a device with no native driver of its own, as long as it offers Modbus TCP or RTU.
Modbus is a simple request and response protocol built around registers. It has no authentication or encryption of its own, so keep Modbus devices on a network segment you control.
Word order and byte order
Modbus registers are 16 bits, so a 32-bit integer or a float spans two of them, and the two ends have to agree on the order. Nexus gives you two independent settings. wordOrder is big or little and says which 16-bit word comes first. byteOrder is the same choice for the bytes inside each word.
Together they cover the four orderings a Modbus device might use. If a float reads as a wild number but its bytes look familiar, change one of these. unitId is the Modbus unit or slave ID.
Register numbering is another trap. Vendor documents disagree on whether the first register is 0 or 1, so check which convention your device manual uses before you assume the address is off by one.
Setup in brief
Choose modbus-tcp or modbus-rtu as the driver, set the endpoint and port (the manual’s example uses port 502), and bind tags in Studio. Like every Nexus device, it carries a name, an endpoint, a port, a fallback pollRateMs, a timeoutMs and a reconnectDelayMs. The name is what tags refer to, so rename a device in the configuration interface rather than by hand in the file, or its tag bindings break. A device holds no address list. It polls exactly the tags bound to it, so adding a tag in Studio starts it scanning without editing the device or restarting Nexus.
The settings are in the manual under Devices and drivers, Modbus. For a serial link, confirm baud rate and parity against your hardware on the trial.
Scan rates and dropped links
Scan rate comes from the tag group, not the device. A group is direct (a fixed rate), leased (quick while something is watching the tag, slow otherwise) or driven (quick while an expression is true). A fresh gateway ships with default-fast at 250 ms, default-slow at 5000 ms and default-historical at 10 s. A tag goes Stale after its rate times staleMultiplier passes without an update, and returns to Good once it is being read again. A rate faster than the device can answer does not bring data sooner; the poll just falls behind.
A device that stops answering is marked disconnected, and its tags follow the quality rules instead of freezing at their last value. Writes forwarded to a device that has since dropped are not silently discarded. The device list in the Nexus configuration section shows each device’s state, with the reason for a failure in the tooltip. The troubleshooting page covers the usual causes.
Licence and trial
Every driver is included in the base licence, which lists at $995. No driver is sold separately, and there is no per-protocol or per-gateway driver charge. An AutomationDirect CLICK is Modbus underneath and has its own CLICK driver with the CLICK address book on top.
The free trial runs for two hours with every driver unlocked, and you can restart it as often as you like. Point it at the controller on your bench and read a tag before you decide anything.
Related drivers
Next step
Read a tag off your own PLC.
The trial is two hours with every driver unlocked, and restartable as often as you like. Point it at the controller on your bench before you decide anything.