ML-OO-MODBUS, Share sensor readings with your SCADA system by MODBUS/RTU

MODBUS auxiliary output board An ML-x17 data logger is designed to operate autonomously and stand-alone at off-grid locations, but we often get requests about the possibility to share readings with a locally present SCADA system.

Because of low-power considerations an ML-x17 goes to sleep after performing sensor readings and can therefore not be interrogated by an external system. To be able to keep the system low power, its possible to equip the ML-x17 with an ultra low power auxiliary output board acting as MODBUS/RTU bridge (slave). This auxiliary output board uses hardly 10uA of idle current and can be woken-up by RS485 communication as well as by the ML-x17. The ML-x17 can be configured to regularly, e.g. after a sensor reading session, update the MODBUS memory map of the board with the latest acquired readings (up to 32 floating-point values). A SCADA system on its turn can regularly, according to its own schedule, wake-up the board to read-out the last know values. When the board is in sleep the first MODBUS request will fail unless preceded (100ms before) with a preamble character, any other bus traffic or retried within the configured inactivity timeout.

Note: If the SCADA systems polls the MODBUS with an interval equal to or lower than the inactivity timeout, the board will not get a chance to fall a sleep and will consume 2.5mA continuously.
MODBUS memory map (Input registers)
AddressData typeDescription
0(30001)WORDNumber of published values
2(30003)DWORDValue validation flags, a value is valid if its corresponding bit is set to 1
4(30005)FLOAT (IEEE-754)The first value
6(30007)FLOAT (IEEE-754)The second value
8(30009)FLOAT (IEEE-754)The third value
66(30067)FLOAT (IEEE-754)The 32nd value