Section 8. Operation
addresses each node, field instruments are effectively multiplexed to a CR800
without additional hardware.
A CR800 goes into sleep mode after 40 seconds of communication inactivity.
Once asleep, two packets are required before the CR800 will respond. The first
packet awakens the CR800; the second packet is received as data. This would
make a Modbus master fail to poll the CR800, if not using retries. The CR800,
through DevConfig or the Status table (see Info Tables and Settings
(p. 527)), can be
set to keep communication ports open and awake, but at higher power usage.
8.10.3.1 Modbus Terminology
Table Modbus to Campbell Scientific Equivalents (p. 437) lists terminology
equivalents to aid in understanding how CR800s fit into a SCADA system.
Modbus to Campbell Scientific Equivalents
Modbus Domain Data Form
Campbell Scientific
Domain
Coils Single bit
Ports, flags, boolean
variables
Digital registers 16 bit word Floating point variables
Input registers 16 bit word Floating point variables
Holding registers 16 bit word Floating point variables
RTU / PLC CR800
Master Usually a computer
Slave Usually a CR800
Field instrument Sensor
8.10.3.1.1 Glossary of Modbus Terms
Term: coils (00001 to 09999)
Originally, "coils" referred to relay coils. In CR800s, coils are exclusively
terminals configured for control, software flags, or a Boolean-variable array.
Terminal configured for control are inferred if parameter 5 of the
ModbusSlave() instruction is set to 0. Coils are assigned to Modbus
registers 00001 to 09999.
Term: digital registers 10001 to 19999
Hold values resulting from a digital measurement. Digital registers in the
Modbus domain are read-only. In the Campbell Scientific domain, the
leading digit in Modbus registers is ignored, and so are assigned together to a
single Dim- or Public-variable array (read / write).
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