Understanding Modbus Terminology; Connecting Modbus Devices; Modbus Client-Server 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 server functionality over other comports use RTU. When acting as a client, the data
logger can be switched between ASCII and RTU protocols using an option in the
ModbusClient()
instruction. See the CRBasic Editor help for detailed instruction information
and program examples:

19.2.3 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 server 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.

19.2.4 Connecting Modbus devices

Data loggers can communicate with Modbus on all available communications 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. 13).
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.

19.2.5 Modbus client-server protocol

Modbus is a client-server protocol. The device requesting the information is called the Modbus
client, and the devices supplying information are Modbus servers. In a standard Modbus
network, there is one client and up to 247 servers. A client does not have a Modbus address.
However, each Modbus server on a shared network has a unique address from 1 to 247.
A single Modbus client device initiates commands (requests for information), sending them to
one or more Modbus server devices on the same network. Only the Modbus client can initiate
communications. Modbus servers, in turn, remain silent, communicating only when responding
to requests from the Modbus client.
https://help.campbellsci.com/crbasic/cr300/ .
19. Communications protocols 113