Delta RMC151 User Manual
Page 444
RMC70/150 and RMCTools User Manual
Entire
Packet
Framing
Header
IP
Header
TCP
Header
Modbus/TCP
Header
Modbus/TCP
Data Area
CRC
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
physically 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.
Supported Protocols
The diagram below demonstrates all protocols supported by the RMC and the layers to
which they belong:
Application Layer
Modbus/TCP,
EtherNet/IP,
PROFINET,
CSP, DMCP,
FINS/UDP
BOOTP
DHCP
Applica
Protoco
Configu
Protoco
Low-lev
Protoco
Transport
Layer
TCP
UDP
Internet Layer
IP (includes ICMP and ARP)
Framing Layer
Ethernet II
Hardware Layer
IEEE 802.3 for 100BaseT
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Delta Computer Systems, Inc.