ML-OO-MODBUS – Option board

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Share sensor readings with your SCADA system by MODBUS/RTU

Our data loggers are 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 the data logger 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 data logger 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 data logger. 

Before you get started

Configuration notes

A data loger 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.

Specifications

  • Protocol: MODBUS/RTU slave (RS-485)
  • Slave address: 1…254 (default: 1)
  • Maximum read size: 32 registers at once
  • Data type register order: Least significant WORD first
  • Baudrate: 4800…38k4bps (default: 19k2)
  • Parity: None
  • Stopbits: 1
  • Inactivity timeout: Infinite, 1…60s (default: 4s)
  • 100uA quiescent current
  • 2.5mA operating current (during MODBUS interrogation)

MODBUS memory map (Input registers)

AddressData typeDescription
0(30001)WORDNumber of published values
1(30002)WORDReserved
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
MODBUS memory map (Input registers)

  • Only values of parameters with their “data output flags” set to the auxiliary output board will appear in the memory map.
  • The order of values in the memory map follows the order of the data logger parameters, which can be organized thru the “Parameter overview”-menu of the data logger.

Discover the possibilities

Want to know more about what YDOC can do to help you with your quest for the right data logger? Do not hesitate to get in contact.

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