Rockwell Automation 1755-OF8 GuardPLC Controller Systems - Manual (Page 152)

152 Publication 1753-UM001C-EN-P - March 2010
Chapter 16 Peer-to-peer Communication Overview
As seen in the figure below, both the HH and the peer-to-peer
protocols are vital for safe Ethernet Communication. HH protocol can
be considered the wire or transport media through which messages
are passed. Peer-to-peer (P2P) is the protocol that runs on the wire,
making sure that the messages are transmitted over the HH
connection within the watchdog time. P2P is the mechanism that
qualifies the GuardPLC Ethernet network as a safety network.
Networking Limitations A peer-to-peer link is defined as communication from one GuardPLC
controller to another GuardPLC controller, or from a GuardPLC
controller to a distributed I/O module. A device on an Ethernet
network must make a connection to another device on the Ethernet
network for the two of them to communicate. Connections need to be
established only between devices that wish to communicate with each
other.
A single GuardPLC controller may have up to 64 connections to other
devices on the GuardPLC Ethernet network (GuardPLC controllers,
GuardPLC distributed I/O module, OPC servers, or programming
terminals). Each connection can transfer up to 900 bytes of data in
each direction (read and write). The data size is determined by the
number of signals transferred between the devices.
In contrast, a GuardPLC distributed I/O module can have only one
connection, the connection to the controller that owns it. The amount
of data shared between a distributed I/O module and the controller is
fixed and defined by the type of I/O module.
The total number of controllers, distributed I/O module, OPC servers,
and programming terminals on a network is limited only by the
number of available IP addresses and the network bandwidth
(maximum 100 Mbps) of a segment of the network. However, large
amounts of data flowing on the network will affect the network
response time, and therefore the safety time of the system.
Ethernet
Controller 2
P2P
COM
CPU
Controller 1
P2P
HH HH
TIP
The peer-to-peer protocol is designated as a safe protocol
according to DIN V 19250 (AK6), IEC61508 (SIL 3) and
ISO 13849-1 (PLe/Cat. 4) respectively.