The Modbus protocol was developed in 1979 by Schneider Automation and is today considered one of the standard protocols for industrial use. The protocol is hierarchically structured and operates in a Master-Slave polling mode. Piciorgros TETRA, GSM, and radio modules support the Modbus protocol in the following variants:  

• Modbus RTU
• Modbus TCP ((not via SDS in TETRA)
• Modbus ASCII (limited)  

Modbus RTU and TETRA:
Many operators of TETRA infrastructures today demand the transmission of their data, measurement values, and control commands via SDS (Short Data Service). The Modbus protocol offers the advantage of being very compact with virtually no overhead, making it particularly suitable for data transmission over the Short Data Service.  

TETRA Network Load When Using the Modbus RTU Protocol:
Piciorgros TRM radio modules (master stations) support the Modbus RTU protocol and feature an automatic Modbus address-to-ISSI converter. This ensures that the dataset is sent directly to a specific end device and is only output at the TETRA base station of the respective Modbus slave station. This eliminates unnecessary high network load, as is the case with transmissions to ISSI groups.  

IP and TETRA:
When referring to IP transmission in TETRA radio networks, it typically involves a computer or PLC with an integrated IP stack that establishes a PPP connection to the TETRA IP world via a TETRA AT modem. However, this shifts some of the radio modem tasks to the automation system, which then also has to handle logging in and out of the radio network, potential modem error messages, stable operation (modem restarts), etc. If a different PLC manufacturer is used in the next project, the entire software package usually needs to be repurchased or rewritten. In contrast, Piciorgros TRM radio modules that support IP provide an IP-Ethernet port for the direct connection of the PLC/PC. The login to the TETRA-IP world via PPP and the IP link setup are automatically handled by the TRM. As a result, the automation system sees the same interface as if it were connected to a wired intranet. 
 

Explanations:

Master Slave Operation
In data transmission protocols, the master is the active station that initiates communication with the slave, sends data to the slave, or retrieves data from the slave. In the Modbus protocol, it is possible to both send data to the slave and retrieve data from the slave with a single data transfer to and from the slave.  

Polling Operation
Polling one station after another. This can, but does not have to, occur cyclically.  

RTU
Remote Terminal Unit, i.e. a remote I/O module. RTU does not stand for Remote TETRA Unit!

TRM
Telemetry Radio Modem, a radio or telemetry modem with serial interface(s).