checksum protection. The packet structure is similar to RTU, but utilizes a different header.
Devices labeled as Modbus gateways will convert from Modbus TCP to Modbus RTU.
Campbell Scientific data loggers support Modbus RTU, Modbus ASCII, and Modbus TCP
protocols. If the connection is over IP, Campbell Scientific data loggers always use Modbus TCP.
Modbus slave functionality over other comports use RTU. When acting as a master, the data
logger can be switched between ASCII and RTU protocols using an option in the
ModbusMaster() instruction. See the CRBasic Editor help for detailed instruction information
and program examples: https://help.campbellsci.com/crbasic/cr6/.
7.2.3 Understanding Modbus Terminology
Many of the object types are named from using Modbus in driving relays: a single-bit physical
output is called a coil, and a single-bit physical input is called a discrete input or a contact.
Information is stored in the slave device in up to four different tables. Two tables store on/off
discrete values (coils) and two store numerical values (registers). The coils and registers each have
a read-only table and read/write table.
7.2.4 Connecting Modbus devices
data loggers can communicate with Modbus on all available communication ports.
Consideration should be given to proper surge protection of any cabled connection. Between
systems of significantly different ground potential, optical isolation may be appropriate. For
additional information on grounds, see Grounds (p. 11).
The common serial interface used for Modbus RTU connections is RS-485 half-duplex, or two-
wire RS-485. This connection uses one differential pair for data, and another wire for a signal
ground. When twisted pair cable is used, the signal can travel long distances. Resistors are often
used to reduce noise. Bias resistors are used to give a clean default state on the signal lines. For
long cable lengths, termination resistors, which are usually 120 ohms, are needed to stop data
corruption due to reflections. Signal grounds are terminated to earth ground with resistors to
prevent ground loops, but allow a common mode signal. The resistors to ground are usually
integral to the equipment. The resistive ground is labeled as RG on Campbell Scientific
equipment.
7.2.5 Modbus master-slave protocol
Modbus is a master-slave protocol. The device requesting the information is called the Modbus
master, and the devices supplying information are Modbus slaves. In a standard Modbus
network, there is one master and up to 247 slaves. A master does not have a Modbus address.
However, each Modbus slave on a shared network has a unique address from 1 to 247.
7. Communications protocols 95
To Next Page
Loading...
