User manual

IEC61131 User and Reference Manual
April 22, 2008
410
A communication error has occurred if the slave does not respond within the expected
maximum time for the complete command and response. The application program is
responsible for detecting this condition. When errors occur, it is recommended that the
application program retry several times before indicating a communication failure.
The completion time depends on the length of the message, the length of the response, the
number of transmitted bits per character, the transmission baud rate, and the maximum
message turn-around time. One to three seconds is usually sufficient. Radio systems may
require longer delays.
Store and Forward Messaging
Store and forward messaging is required on systems where there is no direct link between a
host computer and all the remote sites. This occurs on radio systems where the host
computer transmission cannot be heard by all remote sites. It occurs on systems where one
controller is used as a data concentrator for several remote units. With store and forward
messaging, a request to a controller that cannot be directly accessed by a host is routed
through an intermediate controller, which can communicate with both the host and the
remote controller.
The TeleBUS protocol provides store and forward messaging through address translation. A
controller configured for store and forward operation receives messages destined for a
remote station, re-addresses them according to translation table, and forwards the message
to the remote station. Responses from the remote station are processed in the same
manner.
The TeleBUS protocol allows messages to be re-transmitted on the same port with address
translation. This is used with radio systems. The radio at the intermediate site is used as a
type of repeater. The protocol allows messages to be re-transmitted on a different port, with
or without address translation. This is used where the intermediate controller is a bridge
between two networks.
The TeleBUS protocol driver maintains diagnostics counters at the store and forward site on
the number of messages received and transmitted to aid in the diagnosing of communication
problems.
The protocol cannot support master mode and store-and-forward mode simultaneously on a
serial port. Enabling store and forward messaging disables processing of responses to
master mode commands. Master mode may be used on one port and store-and-forward
mode on another port. Applications requiring both modes on a single port must switch the
modes under control of the application program.
Translation Table
The translation table specifies address and communication port translation. The translation
table differs for SCADAPack and SCADAPack 32 controllers. Each entry in the translation
table for SCADAPack controllers has four components, as shown in the table entry below.
Port A
Station Address A
Port B
Station Address B
The entry defines a bi-directional transfer. A message (poll or reply) received for station A on
port A is re-transmitted to station B on port B. A message received for station B on port B is
re-transmitted to station A on port A.
Each entry in the translation table for SCADAPack 32 controllers has five components, as
shown in the table entry below.
Slave
Interface
Slave
Station
Forward
Interface
Forward
Station
Forward
IP
Address