Issue
Will J-Bus operate with Modbus RTU devices on the same bus?
Product Line
EcoStruxure Building Operation
Environment
- Building Operation Automation Server
- Building Operation Automation Server Premium
- Building Operation Automation Server Bundled
Cause
Compatibility of J Bus required.
Resolution
The J Bus protocol is supported by the Autimation Server, it runs on an RS485 network and uses a subset of the Modbus RTU protocol function codes.
The J Bus devices addressing usually has an offset of 1 compared to the Modbus RTU standard, to apply this offset select "J-Bus Mode" = Yes in the Modbus Master Network Properties.
Verify that the devices to be used have a standard RS485 electrical connection and do not require any special electrical biasing, also ensure that all units connected on the bus have a unique ID/ address number.
If this is the case then you should be able to connect both Modbus RTU and J Bus devices on the same Automation Server port connection but in the Modbus Master Network select the J Bus Mode setting to No.
In J-Bus mode, the Automation Server will automatically reduce the polled register addresses by one, so if the AS is set to Modbus mode, you may need to manually set the register addresses of any J-Bus devices by minus one.
With J Bus as with Modbus sometimes the point addressing needs to be offset by +1 or -1, but check with the 3rd party device supplier/ documentation to determine its specific point addressing requirements.
