Delta RMC101 User Manual download pdf (Page 312)

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RMC100 and RMCWin User Manual

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This diagram shows the four conceptual layers of TCP/IP: application, transport, internet, and

framing. A fifth layer—the hardware layer—is often added below these four layers, but is left out

of this diagram because it is more of a specification of how the data is sent rather than another

protocol header. When a device is sending a packet the packet is assembled from the top layer

down, but when receiving a packet, it must be processed from the bottom layer up.

Here is how the RMC might look at an incoming packet with this structure:

1. Hardware Layer: A full packet is received and passed to the Framing Layer.

2. Framing Layer: The CRC (cyclic redundancy check) is verified. If this fails, the packet is

discarded. Next, the destination MAC address in the framing header is compared with the RMC's

MAC address. If the addresses do not match and the destination address was not a special

broadcast address, the packet is discarded. Otherwise it is passed to the Internet Layer.

3. Internet Layer: The IP address in the IP header is compared with the RMC's user-selectable IP

address. If it does not match, the packet is discarded. Otherwise, it is passed to the Transport

Layer.

4. Transport Layer: The transport layer provides a number of services, but minimally must specify

the port that the data should be sent to. A port is an abstract connection point on a device that

allows for multiple connections to exist on a single device. It also helps determine which

application protocol will follow. The packet may be discarded here too if the destination port is not

one that the RMC supports.

5. Application Layer: In our example, the application protocol is Modbus/TCP, so the Modbus/TCP

header contains data such as the RMC register address to begin reading or writing from, the

number of registers to access, and whether the operation is a read or write. The Modbus/TCP

data area holds the actual words to be written.

Here is a diagram demonstrating all protocols supported by the RMC and the layers to which they

belong:

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