Rev 5.8
SignalFire Telemetry
3
Operation
The Modbus-in-a-Stick supports two modes of operation, a preconfigured register set,
and an automatic scanning “transparent” mode. The Modbus stick can be used in either
mode or a combination depending on the system requirements.
Pre-Configured Register Set Mode
This mode of operation is most useful for large data sets, and frequent polling of a set register map.
This mode requires that the Modbus Stick be configured with the register map with the configuration
utility at the time of installation.
In this mode the pre-configured set of registers is automatically read from the Modbus sensor device
and forwarded to the Modbus gateway on a pre-defined schedule (1 minute to 5 minutes is typical).
The register data is then buffered in the gateway and is available to be read by the RTU at any time.
This is the most efficient and robust mode of operation.
Transparent Modbus Mode (version Modbus_r38 and Gateway version 7.37 and later only)
This mode requires no Modbus setup at the Modbus Stick, and can be used to smaller number of
registers that only need to be read or written infrequently. All standard Modbus opcodes are
supported.
Upon initial power-up the Modbus Stick will automatically poll all slave IDs (1-240) to discover
attached devices. Any devices found will be reported to the gateway so that a wireless link will exist to
the Modbus device. This scan is automatically repeated every hour in the event that an additional
device is added to the bus. The scan may also be initiated from the Modbus Stick’s debug port, or
remotely from the Modbus Gateway. See the Modbus Gateway manual for register details.
Alternatively one or more Modbus Program steps can be entered for a given slave ID so that the Slave
ID will be known to the Gateway.
When the RTU polls the gateway for a Modbus register, if the register is buffered (meaning it was pre-
configured) the buffered value is returned. If the register value is not buffered, but the Modbus slave
ID is known, the request is forwarded over the SignalFire wireless network to the Modbus sensor, the
response is forwarded back to the gateway and delivered to the RTU. Due to the multi-hop wireless
network, latency will be introduced. It is required that the RTU’s timeout be on the order of 5-10
seconds to allow for maximum possible networks delays. This limits the effective amount of data that
can be pulled.
