Rockwell Automation T8xxx Trusted Communications Interface - Manual (Page 21)

Trusted
TM
Communication Interface T8151B
Issue 21 Apr 10 PD-T8151B 21
3.4.1. Connection Timeout
‘Connection Timeout’ should be left at the default of ‘Disconnect after 300 seconds’. This allows
Ethernet-based slaves to recover properly after a lost connection. 300 seconds is appropriate for
communications where cable breaks are not normally expected (e.g. not involving a modem or other
link likely to disconnect).
The TCP/IP stack does not detect the loss of Modbus communications caused by a pulled connection
or cable break. A broken connection will not be seen by the TCP/IP stack, and will be retained as a
valid connection. When the broken connection is recovered, the Modbus Master may resume
communications using the existing connection, but it is more likely to establish a new connection. Each
re-connection consumes one of the available connections. If this occurs on a regular basis, all the
available connections will be consumed and no further connection will be possible. On early
communication interfaces that do not recognise the connection timeout setting, removing and re-
inserting the communication interface and rebooting it will recover the available connections.
3.4.2. SOE over Modbus
The ‘SOE over Modbus’ protocol allows buffered transfer of events via a communication interface
Modbus slave. This protocol provides access to the SOE buffer events (but not Process Historian),
over a Modbus data link, for where the default OPC server/SOE collector protocols prove too slow.
However, SOE over Modbus protocol requires an application in the Modbus Master (remote end) to
read, interact with and interpret the buffer, and assign event data to variables. For more information on
events and their configuration, see product description PD-8013.
SOE over Modbus is implemented as a service running on a communication interface. When enabled,
the service manages a “window” of Modbus addresses that implement the protocol, which is
automatically available on all ports of that module which are configured as Modbus slaves.
The settings in the Modbus Slave Configuration window define a block of register addresses which
provide a window for accessing events.
Selecting the “Enable” check box enables the service. Once enabled, the Base Address and Block
Count fields will become active.
Base Address Field:
This field specifies that starting address of the window that the protocol will use. It can be located at
any valid Modbus holding register address (i.e. Modbus addresses in the 40,000 to 49,984 range). As
this field is modified, the End Address field will be updated.
Block Count Field:
This specifies the number of events that can be read by the Modbus master in a single Modbus read.
Each event block requires 4 Modbus holding register addresses. As the Block Count field is modified,
the End Address field will be updated. The Event Block Count must specify at least 2 event blocks.
End Address Field:
The End Address field specifies the last address of the Modbus address “window” that the SOE Over
Modbus protocol will use. It is calculated as:
start address as specified in the Base Address field
+ 7 (for protocol overhead)
+ (the number of blocks as specified in the Block Count field * 4)
The Modbus address “window” as indicated by the Base Address and End Address fields should be
reserved exclusively for use by the SOE over Modbus service. Defining a variable with a conflicting
Modbus address may lead to loss of events, corrupted event data, or protocol errors that prevent the
continued operation of the SOE over Modbus service.
The SOE over Modbus protocol and data block format are described in appendix 7 of this document.