Understanding Modbus Terminology; Connecting Modbus Devices; Modbus Master-Slave Protocol - Campbell CR300 Series Product Manual

checksum protection. The packet structure is similar to RTU, but uses 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()
and program examples:

15.2.3 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.

15.2.4 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
The data logger communicates with Modbus over RS-232, or RS-485 using a RS-232-to RS-485
adapter and over TCP using an Ethernet or wireless connection .

15.2.5 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.
A single Modbus master device initiates commands (requests for information), sending them to
one or more Modbus slave devices on the same network. Only the Modbus master can initiate
communications. Modbus slaves, in turn, remain silent, communicating only when responding to
requests from the Modbus master.
Every message from the master will begin with the slave address, followed by the function code,
function parameters, and a checksum. The slave will respond with a message beginning with its
address, followed by the function code, data, and a checksum. The amount of data in the packet
instruction. See the CRBasic Editor help for detailed instruction information
https://help.campbellsci.com/crbasic/cr300/.
Grounds (p. 10).
15. Communications protocols 90