Packet Structure Comparison of Modbus RTU, Modbus TCP and Modbus RTU over TCP

Issue
How does the packet structure of Modbus RTU, Modbus TCP and Modbus RTU over TCP differ?

Product Line
Modbus RTU, Modbus TCP

Environment
Modbus Communications

Cause
Modbus TCP and Modbus RTU packets differ in format.

Resolution
The image below and attached document gives a brief diagrammatic description of the three:


NOTE:  Modbus RTU packets cannot be transferred on a Modbus TCP frame when the device's Serial Port does not have access to the device's Ethernet port.  For example, there is a chain of Modbus Master Controller > Serial RS-485 > PM8000 Slave > Ethernet Cat5 > PM5500 Slave.  The Serial Slave PM8000 cannot redirect a Modbus RTU packet from a Modbus Matser Controller to the PM5500 Slave, because the PM8000's RS-485 port does not have access to the Ethernet port.